Test case and m4a file for BGO #701689 and BGO #701690

I've added a test case and file for two bugs I've recently filed. They are related to reading MusicBrainz tags from m4a files.

It appears the tags retain a beginning null character (\0) which causes all tag values to be returned as null, also the MusicBrainzTrackId is reading the wrong tag name.

Please see the bug reports for more details:

 * https://bugzilla.gnome.org/show_bug.cgi?id=701689
 * https://bugzilla.gnome.org/show_bug.cgi?id=701690

Thanks in advance.