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

There are two exceptions where a migration was not possible as the tests rely on Rule implementations that have not (yet) been migrated to JUnit5:

• SupportAutomatedBundleConfigurationSystemPropertiesTest using RealJenkinsRule
• SlaveLaunchLogsTest using InboundAgentRule

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.

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

[strangelookingnerd]

Looking at the failed tests I'm a little confused to be honest. Not sure how my changes could have affected the test results here. I will try to investigate a little further.

[jglick]

Tests in this plugin, especially on Windows, are in a state of flux given the recent infrastructure changes. I would not advise any refactorings on this plugin’s tests until the existing tests are stable.

(I also personally do not plan to spend any time reviewing JUnit 5 migration PRs. If there are any other active maintainers, they can feel free.)