Ban JUnit 4 imports

[strangelookingnerd]

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.

Testing done

None. Rely on ci.jenkins.io.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.99%. Comparing base (803edab) to head (c1a339f).
⚠️ Report is 62 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #463      +/-   ##
============================================
+ Coverage     71.80%   71.99%   +0.18%     
- Complexity      162      163       +1     
============================================
  Files            16       16              
  Lines           532      532              
  Branches         51       51              
============================================
+ Hits            382      383       +1     
+ Misses          124      123       -1     
  Partials         26       26              

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