added composer_sort on test_mediafile.py since there is artist_sort
added composer_sort on test_mediafile.py since there is artist_sort

ibmibmibm/beets,beetbox/beets,sampsyo/beets,shamangeorge/beets,pkess/beets,beetbox/beets,ibmibmibm/beets,sampsyo/beets,ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,SusannaMaria/beets,shamangeorge/beets,beetbox/beets,pkess/beets,SusannaMaria/beets,sampsyo/beets,SusannaMaria/beets,beetbox/beets,ibmibmibm/beets,jackwilsdon/beets,pkess/beets,shamangeorge/beets,SusannaMaria/beets,shamangeorge/beets,jackwilsdon/beets,sampsyo/beets,pkess/beets