playback: Remove skipped track on next in consume mode

Also adds core level tests of consume behavior on next/prev/eot.

Fixes #902
Merge pull request #919 from jodal/fix/902-consume-track-on-next

playback: Remove skipped track on next in consume mode
Merge branch 'release/v0.19.x' into develop

Conflicts:
	mopidy/core/playback.py