Migrate all tests from JUnit 4 to JUnit 5

[marecabo]

This change removes all JUnit 4 methods and replaces them with their JUnit 5 counterparts.

[kt86]

@marecabo Very nice step to update all the things here! Thank you very much!! I assume that you have some kind of auto-formatter active, which reformats at least comments (?) I have no issues with it, but one should see, that it does not end in something that your IDE change it to a) and another ones IDE to b),.... leading to many formatting changes w/o any changes in functionality and thus hiding the latter ones....