Skip unloadable tracks in the AutoDJ queue. Fixes Bug #1396931.

* On track load failed remove the failed track from the queue and load
  the next track from the queue to the deck.
* Update existing on-enable test cases to check what happens after
  receiving track load success signals.
* Add test case for ADJ_ENABLE_P1LOADED state failing to load the
  first track.
* Add test case for ADJ_ENABLE_P1LOADED state failing to load the
  second track after successfully loading the first track.
* Add test cases for single-deck-playing AutoDJ enable failing to load a
  track in the non-playing player.
* Add test cases for P1FADING / P2FADING mode successfully loading a
  track and failing to load a track.
Add a note to prevent removal of a redundant looking set.