refactor: Remove public visibility of JUnit 5 tests

[timtebeek]

Another round of removing public modifiers from tests not needed anymore in JUnit JUpiter, after #3630 (comment).
This helps the public modifiers that are there for a reason stand out, and stops proliferation elsewhere.

Probably best to squash merge this one; had to revert some formatting changes.

[timtebeek]

There's also some common static analysis issues that could be cleaned up, but I was doubting if that's appreciated as it does switch some stylistic choices perhaps.

[codefromthecrypt]

Thanks yet again. We always squash on way in if multiple commits, so no problem there.

[timtebeek]

And of course if you'd want to run any of these same recipes against other openzipkin repositories I can facilitate that through the platform; just let me know and I can setup an OpenZipkin organization to select there.

[codefromthecrypt]

@timtebeek wrt OpenZipkin organization. Thanks, it would be helpful. Once we release 2.25.0 here we'll go to zipkin-reporter-java and do the same things notably off junit 4. Moderne is already proving a good decrufting tool, particularly as we are free to use offline and leave no side effects in the build.

[timtebeek]

Ah perfect, you can now select OpenZipkin from top left corner and run recipes against all of these repositories

Here's first run of JUnit Jupiter best practices against among others the zipkin-reporter-java that you mentioned:
https://app.moderne.io/results/YiPSV5epL

You can then follow up with the AssertJ best practices, Logging best practices and any other recipes you feel might improve the code there. Hope that helps!

[codefromthecrypt]

Thanks @timtebeek and just in time!