Merge: Popcorn: clean tests

This PR does two things:
* clean the lib by moving all the tests to `tests/` directories, easier to see what is important, great visual improvement for the examples.
* use a lot of imagination to avoid the `class name X already used`. It mostly replaces Short&Cool names by LooooonnggggandAndBooooringNameForSoSimpleHandler. No more warnings when make checking.

Pull-Request: #2434
Reviewed-by: Jean Privat <jean@pryen.org>