Pull the whole "Testing" documentation section up to the top level

Closes gh-38361

Author: mhalbritter

spring-boot-project/spring-boot-docs/src/docs/antora/modules/ROOT/pages/redirect.adoc

@@ -218,4 +218,4 @@ For the example above, setting this property to `baggage1` results in an MDC ent
 == Tests
 
 Tracing components which are reporting data are not auto-configured when using `@SpringBootTest`.
-See xref:features/testing.adoc#features.testing.spring-boot-applications.tracing[the testing section] for more details.
+See xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.tracing[the testing section] for more details.

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/actuator/tracing.adoc

@@ -286,7 +286,7 @@ spring:
 [[features.dev-services.testcontainers]]
 == Testcontainers Support
 
-As well as xref:features/testing.adoc#features.testing.testcontainers[using Testcontainers for integration testing], it's also possible to use them at development time.
+As well as xref:testing/testcontainers.adoc#testing.testcontainers[using Testcontainers for integration testing], it's also possible to use them at development time.
 The next sections will provide more details about that.
 
 
@@ -312,7 +312,7 @@ To do this, you need to make sure that the `spring-boot-testcontainers` module h
 Once that has been done, you can create a `@TestConfiguration` class that declares `@Bean` methods for the containers you want to start.
 
 You can also annotate your `@Bean` methods with `@ServiceConnection` in order to create `ConnectionDetails` beans.
-See xref:features/testing.adoc#features.testing.testcontainers.service-connections[the service connections] section for details of the supported technologies.
+See xref:testing/testcontainers.adoc#testing.testcontainers.service-connections[the service connections] section for details of the supported technologies.
 
 A typical Testcontainers configuration would look like this:
 
@@ -338,7 +338,7 @@ TIP: You can use the Maven goal `spring-boot:test-run` or the Gradle task `bootT
 ==== Contributing Dynamic Properties at Development Time
 
 If you want to contribute dynamic properties at development time from your `Container` `@Bean` methods, you can do so by injecting a `DynamicPropertyRegistry`.
-This works in a similar way to the xref:features/testing.adoc#features.testing.testcontainers.dynamic-properties[`@DynamicPropertySource` annotation] that you can use in your tests.
+This works in a similar way to the xref:testing/testcontainers.adoc#testing.testcontainers.dynamic-properties[`@DynamicPropertySource` annotation] that you can use in your tests.
 It allows you to add properties that will become available once your container has started.
 
 A typical configuration would look like this:
@@ -365,7 +365,7 @@ To do so, add the `@ImportTestcontainers` annotation to your test configuration
 
 include-code::MyContainersConfiguration[]
 
-TIP: If you don't intend to use the xref:features/testing.adoc#features.testing.testcontainers.service-connections[service connections feature] but want to use xref:features/testing.adoc#features.testing.testcontainers.dynamic-properties[`@DynamicPropertySource`] instead, remove the `@ServiceConnection` annotation from the `Container` fields.
+TIP: If you don't intend to use the xref:testing/testcontainers.adoc#testing.testcontainers.service-connections[service connections feature] but want to use xref:testing/testcontainers.adoc#testing.testcontainers.dynamic-properties[`@DynamicPropertySource`] instead, remove the `@ServiceConnection` annotation from the `Container` fields.
 You can also add `@DynamicPropertySource` annotated methods to your declaration class.
 
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/dev-services.adoc

@@ -24,7 +24,7 @@ Sources are considered in the following order:
 . Properties from `SPRING_APPLICATION_JSON` (inline JSON embedded in an environment variable or system property).
 . Command line arguments.
 . `properties` attribute on your tests.
-  Available on xref:api:java/org/springframework/boot/test/context/SpringBootTest.html[`@SpringBootTest`] and the xref:features/testing.adoc#features.testing.spring-boot-applications.autoconfigured-tests[test annotations for testing a particular slice of your application].
+  Available on xref:api:java/org/springframework/boot/test/context/SpringBootTest.html[`@SpringBootTest`] and the xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.autoconfigured-tests[test annotations for testing a particular slice of your application].
 . {url-spring-framework-javadoc}/org/springframework/test/context/DynamicPropertySource.html[`@DynamicPropertySource`] annotations in your tests.
 . {url-spring-framework-javadoc}/org/springframework/test/context/TestPropertySource.html[`@TestPropertySource`] annotations on your tests.
 . xref:using/devtools.adoc#using.devtools.globalsettings[Devtools global settings properties] in the `$HOME/.config/spring-boot` directory when devtools is active.

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/external-config.adoc

@@ -145,7 +145,7 @@ JUnit 5 enables a test class to be instantiated once and reused for all of the c
 This makes it possible to use `@BeforeAll` and `@AfterAll` annotations on non-static methods, which is a good fit for Kotlin.
 
 To mock Kotlin classes, https://mockk.io/[MockK] is recommended.
-If you need the `MockK` equivalent of the Mockito specific xref:features/testing.adoc#features.testing.spring-boot-applications.mocking-beans[`@MockBean` and `@SpyBean` annotations], you can use https://github.com/Ninja-Squad/springmockk[SpringMockK] which provides similar `@MockkBean` and `@SpykBean` annotations.
+If you need the `MockK` equivalent of the Mockito specific xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.mocking-beans[`@MockBean` and `@SpyBean` annotations], you can use https://github.com/Ninja-Squad/springmockk[SpringMockK] which provides similar `@MockkBean` and `@SpykBean` annotations.
 
 
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/features/kotlin.adoc

@@ -58,7 +58,7 @@ For example, you might choose to run native tests once a day.
 Spring Framework includes ahead-of-time support for running tests.
 All the usual Spring testing features work with native image tests.
 For example, you can continue to use the `@SpringBootTest` annotation.
-You can also use Spring Boot xref:features/testing.adoc#features.testing.spring-boot-applications.autoconfigured-tests[test slices] to test only specific parts of your application.
+You can also use Spring Boot xref:testing/spring-boot-applications.adoc#testing.spring-boot-applications.autoconfigured-tests[test slices] to test only specific parts of your application.
 
 Spring Framework's native testing support works in the following way:
 

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/native-image/testing-native-applications.adoc

@@ -0,0 +1,30 @@
+[[testing]]
+= Testing
+
+Spring Boot provides a number of utilities and annotations to help when testing your application.
+Test support is provided by two modules: `spring-boot-test` contains core items, and `spring-boot-test-autoconfigure` supports auto-configuration for tests.
+
+Most developers use the `spring-boot-starter-test` "`Starter`", which imports both Spring Boot test modules as well as JUnit Jupiter, AssertJ, Hamcrest, and a number of other useful libraries.
+
+[TIP]
+====
+If you have tests that use JUnit 4, JUnit 5's vintage engine can be used to run them.
+To use the vintage engine, add a dependency on `junit-vintage-engine`, as shown in the following example:
+
+[source,xml]
+----
+<dependency>
+	<groupId>org.junit.vintage</groupId>
+	<artifactId>junit-vintage-engine</artifactId>
+	<scope>test</scope>
+	<exclusions>
+		<exclusion>
+			<groupId>org.hamcrest</groupId>
+			<artifactId>hamcrest-core</artifactId>
+		</exclusion>
+	</exclusions>
+</dependency>
+----
+====
+
+`hamcrest-core` is excluded in favor of `org.hamcrest:hamcrest` that is part of `spring-boot-starter-test`.

spring-boot-project/spring-boot-docs/src/docs/antora/modules/reference/pages/testing/index.adoc

@@ -0,0 +1,14 @@
+[[testing.spring-applications]]
+= Testing Spring Applications
+
+One of the major advantages of dependency injection is that it should make your code easier to unit test.
+You can instantiate objects by using the `new` operator without even involving Spring.
+You can also use _mock objects_ instead of real dependencies.
+
+Often, you need to move beyond unit testing and start integration testing (with a Spring `ApplicationContext`).
+It is useful to be able to perform integration testing without requiring deployment of your application or needing to connect to other infrastructure.
+
+The Spring Framework includes a dedicated test module for such integration testing.
+You can declare a dependency directly to `org.springframework:spring-test` or use the `spring-boot-starter-test` "`Starter`" to pull it in transitively.
+
+If you have not used the `spring-test` module before, you should start by reading the {url-spring-framework-docs}/testing.html[relevant section] of the Spring Framework reference documentation.