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I think, this "Part 2" is ready now. It's very big, but all changes follow a simple theme:

• Use JUnit5 Test annotation
• Use static imports for Assertions (and sometimes adjust the parameter order, e.g. the message parameter is different in JUnit5)
• Make test classes and methods package private
• Add dependency for junit-jupiter and system-lambda, where needed

There are no function changes (at least, there should be no...).

I didn't remove the old junit dependencies yet, that's a task for the next part (see tasks in #3797).
Also base tests for CLI, RuleSetFactory and LanguageVersions are not migrated yet and will be my
next task.