Clean up and simplify tests

[friederbluemle]

This is a major cleanup of all test code. It's a follow-up to the nice changes and already merged #555 by @ming13, and also includes a few lines from #556 (which will only contain the visibility fixes after rebasing it on top of this PR).

Since there are a lot of lines changes, I'll summarize it here:

The PR:

• Does NOT affect any production code (no changes in src)
• Does NOT move the test files or change the test setup (yet) - tests still have to be run in instrumented mode

The most important points this PR addresses:

• Shorter, more idiomatic assert usage (e.g. assertEquals instead of Assert.assertEquals)
• Fixed the order of all assertion parameters: The expected value is the first parameter, but in most assertions were written the opposite way
• Minor (automatic) formatting cleanups
• Java language simplifications (e.g. "Explicit type can be replaced with <>")

I have local wip branch that allows me to run the tests on the JVM, and I'll open a PR after this one.

[jpoehnelt]

LGTM, thanks for helping clean up!