Upgrade tests to JUnit5

[strangelookingnerd]

Upgrades all tests to run with JUnit5. Notable changes besides the obvious:

• DataPoint based Theory tests now use ParameterizedTest with a MethodSource or EnumSource as appropriate.
• Rules AbstractSampleRepoRule and AbstractSampleDVCSRepoRule were not upgraded as they are used by other plugin. Consequently I left the dependency to JUnit4 in pom.xml be. If desired I can look into upgrading them, too.

Testing done

Ran mvn clean verify before and after changes. While all are successful, the overall number of reported tests increased from 124 to 135 due to the way ParameterizedTest work.

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

[strangelookingnerd]

@jenkinsci/scm-api-plugin-developers Kindly requesting a review.