Media Kit: Add MP3 audio stream decoder test.

- This test currently fails and thus showing that there is something wrong in
  the Media Kit.
- This test was first developed and tested on Haiku R1 Alpha 4.1 where
  every thing was hacked together to make this test succeed, thus showing that
  the current Media Kit state of affair has some issues and not this test :)
- The test comes with documentation (@see mp3_decoder_test.cpp).

Signed-off-by: Colin Günther <coling@gmx.de>