fix for filesystem case sensitivity

My Mac filesystem is case-insensitive. The test was looking for
'getinfo_artist_metallica.xml'; so on the mac when it finds
'getinfo_artist_Metallica.xml' it works fine; on a case-sensitive filesystem,
not so much.

Changed the artist that test looks for and added the new xml fixture.
Merge branch 'mbid-lookup-fix'

* mbid-lookup-fix:
  fix for filesystem case sensitivity