chore: stabilizing DefaultSupervisingRouteControllerTest

[apupier]

Previous improvement of assertion was when the list of failure was not arriving always in the same order. But the test is still flaky, there are 2 reasons:

• the exception was not set yet as all retries were not finished
• the lists used in test to collect events and failures were not synchronized and can be called in different threads. It seems that it was causing some elements to not be added correctly. I suspected a caught ConcurrentModificationException but in debug mode it is not hit. So not sure the root reason using syncehonizedList seems to do the trick.
  Using @RepeatedTest locally, I had between 5 and 15 tests in error on 50 attempts before. Now I hit 100% success.

Description

last falky failure on Ci https://ci-builds.apache.org/job/Camel/job/Camel%20Core%20(Build%20and%20test)/job/main/lastCompletedBuild/testReport/org.apache.camel.impl.engine/DefaultSupervisingRouteControllerTest/BuildAndTest___Matrix___JDK_NAME____jdk_21_latest___PLATFORM____ubuntu_avx____Test___testSupervising/

org.opentest4j.AssertionFailedError: expected: not <null>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:152)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertNotNull.failNull(AssertNotNull.java:49)
	at org.junit.jupiter.api.AssertNotNull.assertNotNull(AssertNotNull.java:35)
	at org.junit.jupiter.api.AssertNotNull.assertNotNull(AssertNotNull.java:30)
	at org.junit.jupiter.api.Assertions.assertNotNull(Assertions.java:304)
	at org.apache.camel.impl.engine.DefaultSupervisingRouteControllerTest.testSupervising(DefaultSupervisingRouteControllerTest.java:98)
	at java.base/java.lang.reflect.Method.invoke(Method.java:580)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:387)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1312)
	at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1843)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1808)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:188)

Target

• I checked that the commit is targeting the correct branch (note that Camel 3 uses camel-3.x, whereas Camel 4 uses the main branch)

Tracking

• If this is a large change, bug fix, or code improvement, I checked there is a JIRA issue filed for the change (usually before you start working on it).

Apache Camel coding standards and style

• I checked that each commit in the pull request has a meaningful subject line and body.

• I have run mvn clean install -DskipTests locally from root folder and I have committed all auto-generated changes.

[github-actions]

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