Migrate tests to JUnit5

[strangelookingnerd]

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

• Migrate annotations
• Migrate assertions
• Minor cleanup
• Remove public visibility of test classes and methods

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[strangelookingnerd]

@jenkinsci/text-finder-plugin-developers Kindly requesting a review.

[basil]

I would prefer to wait until jenkinsci/throttle-concurrent-builds-plugin#302 (review) is addressed before migrating my plugins to JUnit 5. That way, if any API changes are needed in order to address that comment, I would only need to perform them once.