Removing deprecated TestRule in TestPipeline

[scott-strong]

addresses #30767

As the repository no longer supports junit 4 in favor of junit 5, the TestPipeline needed to be updated in order to remove the deprecated TestRule in favor of the extension interfaces BeforeEachCallback and AfterEachCallback. This essentially achieves the same purpose of running code before and after a test statement, but it then required some small tweaks to the code.

Note: I wasn't sure what error messages to use when checking for values that should almost always be set like the test method, test class, and current element.

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[myhau]

Hello @scott-strong

I'm exploring the possibility of using JUnit 5 (instead of 4) to test Apache Beam pipelines (#18733).

Just curious, why was this closed?

[scott-strong]

@myhau I think I had issues with some of the test pipelines, and I was able to fix my issue by including both junit4 and junit5 as dependencies in my repository (which is still not ideal).

I think the changes I made were valid, I just didn't feel strongly enough to continue pushing them through.