Properly subscribe to CloseableConnectionFactory#close

Closes gh-25565

Author: snicoll

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/metrics/r2dbc/ConnectionPoolMetricsTests.java

@@ -59,7 +59,7 @@ void init() {
 	@AfterEach
 	void close() {
 		if (this.connectionFactory != null) {
-			this.connectionFactory.close();
+			StepVerifier.create(this.connectionFactory.close()).verifyComplete();
 		}
 	}
 

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/r2dbc/ConnectionFactoryHealthIndicatorTests.java

@@ -59,7 +59,7 @@ void healthIndicatorWhenDatabaseUpWithConnectionValidation() {
 			}).verifyComplete();
 		}
 		finally {
-			connectionFactory.close();
+			StepVerifier.create(connectionFactory.close()).verifyComplete();
 		}
 	}
 
@@ -110,7 +110,7 @@ void healthIndicatorWhenDatabaseUpWithSuccessValidationQuery() {
 			}).verifyComplete();
 		}
 		finally {
-			connectionFactory.close();
+			StepVerifier.create(connectionFactory.close()).verifyComplete();
 		}
 
 	}
@@ -130,7 +130,7 @@ void healthIndicatorWhenDatabaseUpWithFailureValidationQuery() {
 			}).verifyComplete();
 		}
 		finally {
-			connectionFactory.close();
+			StepVerifier.create(connectionFactory.close()).verifyComplete();
 		}
 	}