Migrate tests to JUnit5

[strangelookingnerd]

This PR aims to migrate all tests to JUnit5. Changes include:

• Migrate annotations and imports
• Migrate assertions
• Remove public visibility for test classes and methods
• Minor clean up

I am well aware that this is a quite large changeset however I hope that there is still interest in this PR and it will be reviewed.
If there are any questions, please do not hesitate to ping me.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

[strangelookingnerd]

For some reason ReaperTest.testStopWatchingOnCloseException is failing.

org.opentest4j.AssertionFailedError: no watcher events ==> expected: <0> but was: <2>
	at org.junit.jupiter.api.AssertionFailureBuilder.build(AssertionFailureBuilder.java:151)
	at org.junit.jupiter.api.AssertionFailureBuilder.buildAndThrow(AssertionFailureBuilder.java:132)
	at org.junit.jupiter.api.AssertEquals.failNotEqual(AssertEquals.java:197)
	at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:150)
	at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:563)
	at org.csanchez.jenkins.plugins.kubernetes.pod.retention.ReaperTest$CapturingReaperListener.expectNoEvents(ReaperTest.java:855)
	at org.csanchez.jenkins.plugins.kubernetes.pod.retention.ReaperTest.testStopWatchingOnCloseException(ReaperTest.java:371)
	at java.base/java.lang.reflect.Method.invoke(Method.java:569)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)
	at java.base/java.util.ArrayList.forEach(ArrayList.java:1511)

I also see this test failing locally when running the class. Running the test alone however works fine. Adding await() or Thread.sleep did not help. I am kind of lost here to be honest.

[Vlatombe]

For some reason ReaperTest.testStopWatchingOnCloseException is failing.

Addressed

[Vlatombe]

👍