test_fetchart: Avoid duplicate code in future tests.
test_fetchart: Avoid duplicate code in future tests.

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