MediaEngine: null pending_track to not highlight invalid track (bgo#722731)

MediaEngine.PlayerEngine.SetNextTrack() was filling the private field
'pending_track' with a value that would be, a bit later, retrieved
by OnEventChanged(), to assign its value to the more important
'current_track' whenever a StartOfStream event is received.

The problem that this had is in the case of a track whose playback
generated an error: a PlayerEvent.Error was emitted, but nobody was
subscribed to it to nullify 'pending_track' accordingly.

The visual consequence of this was that if playback stopped due to
an error, and the user tried manually to start playback again by
choosing, this time, a valid track, the list view would highlight
the previously-failed track as it was the one currently played.

The way to fix it is to nullify the 'pending_track' when a manual
action is pursued by the user, that is, when
MediaEngine.PlayerEngine.Open() is called.

As this behaviour is very subtle and a 1-liner bugfix like this is
so easy to break again, a NUnit test is added as well, that covers
this scenario. (And in regards to this, the AssertTransition() call
to check the events of service.Dispose() action had to be moved to
the TearDown nunit step, otherwise we couldn't have reused the
PlayerEngineService instance and other infrastructure that the
TestFixtureSetUp is creating.)