Migrate remaining tests to JUnit5 and ban JUnit4 imports

[strangelookingnerd]

This PR aims to migrate the last remaining tests to JUnit5. Changes include:

• Migrate annotations and imports
• Migrate assertions
• Remove public visibility for test classes and methods
• Minor clean up
• Ban JUnit 4 imports

To prevent regressions when adding new tests, jenkinsci/plugin-pom#1178 introduced a new flag that enables a Maven Enforcer rule banning org.junit.* imports while allowing org.junit.jupiter.*.

With this change, the build will fail if any org.junit.* imports are introduced.

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]

Failing test seems to be caused by infrastructure. Runs fine on my machine.

[strangelookingnerd]

@jenkinsci/credentials-plugin-developers Kindly requesting a review.