Migrate tests to JUnit5

[strangelookingnerd]

This PR aims to migrate all tests to JUnit5. Changes include:

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

I am well aware that this is a quite large changeset however I hope that there is still interest in this PR and it will be reviewed. If there are any questions, please do not hesitate to ping me.

• [x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• [x] Ensure that the pull request title represents the desired changelog entry
• [x] Please describe what you did
• [x] Link to relevant issues in GitHub
• [x] Link to relevant pull requests, esp. upstream and downstream changes
• [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue

[strangelookingnerd]

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