`mopidy local scan` doesn't have any tests

[adamcik]

While reviewing mopidy/mopidy#1555 I realized that mopidy.local.commands.ScanCommand doesn't have tests. We should split out everything in the run() command and test it there or see about testing it directly.

Though given mopidy/mopidy#1563 this might be something to move over to the new project once it's up.

[bernd-wechner]

For the record: see mopidy/mopidy#1003 (not mopidy/mopidy#1563)