core: Test next/prev skips over unplayable tracks
core: Test next/prev skips over unplayable tracks

Fixes #1418

Based on tests that was present in 1.1.2 but dropped in the #1400 merge.

jcass77/mopidy,jodal/mopidy,kingosticks/mopidy,kingosticks/mopidy,ZenithDK/mopidy,vrs01/mopidy,vrs01/mopidy,vrs01/mopidy,adamcik/mopidy,adamcik/mopidy,ZenithDK/mopidy,mopidy/mopidy,jodal/mopidy,jcass77/mopidy,mopidy/mopidy,ZenithDK/mopidy,ZenithDK/mopidy,adamcik/mopidy,jcass77/mopidy,mopidy/mopidy,jodal/mopidy,kingosticks/mopidy,vrs01/mopidy