Ban junit4 imports

[kamphaus]

Following the instructions from jenkinsci/plugin-pom#1178 we enforce the ban on junit4 imports.

Testing done

• Executed builds to ensure tests/packaging keeps working with ban-junit4 enforced

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 → NA
• 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 → NA