GH-9878: Fix ConcurrentModificationEx for SpringIntegrationTestExecutionListener

Fixes: #9878
Issue link: #9878

* `return Collections.unmodifiableList(this.autoStartupCandidates);` in the `MockIntegrationContext`

Signed-off-by: Alexander Hain <Alexander.hain@cloudflight.io>

[artem.bilan@broadcom.com Improve commit message]

**Auto-cherry-pick to `6.4.x` & `6.3.x`**

Signed-off-by: Artem Bilan <artem.bilan@broadcom.com>

Author: proggler23

spring-integration-test/src/main/java/org/springframework/integration/test/context/MockIntegrationContext.java

@@ -19,6 +19,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
 import java.util.List;
 
 import reactor.core.publisher.Mono;
@@ -59,6 +60,7 @@
  *
  * @author Artem Bilan
  * @author Yicheng Feng
+ * @author Alexander Hain
  *
  * @since 5.0
  *
@@ -111,7 +113,7 @@ public void afterSingletonsInstantiated() {
 	}
 
 	List<AbstractEndpoint> getAutoStartupCandidates() {
-		return this.autoStartupCandidates;
+		return Collections.unmodifiableList(this.autoStartupCandidates);
 	}
 
 	/**