convert mediaDescriptions test to cppunit.
Register media add-ons on BMediaFormat::GetFormatFor().

* Fixes #11018.
* In this case the FormatManager format list is accessed without going
throught the AddOnManager, so we must tell the AddOnManager to register
the add-ons (as this is what populates the FormatManager format list).
* Remove now unneeded workaround in mp3_decoder_test.