Migrate GitSampleRepoRule to JUnit5

[strangelookingnerd]

In #443 I said that TestProjectObjectIsPresentOnMultibranchWorkflow could not be migrated to JUnit5 as it is using GitSampleRepoRule. Turns out I was wrong about that and thanks to jenkinsci/git-plugin#1574 it can be made compatible with JUnit5 as well.

Testing done

mvn clean verify

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 demonstrates feature works or fixes the issue

[kinow]

Thanks (again)!