Update MusicBrainz to support writing the individual artist names to the tags.
This helps clients which scan MB tagged music in an offline mode to correctly index the artists until they can be resolved online.
contributions by zas: Reduce code redundancy as suggested by @mwiencek, add a test case
Iterate only once over node in artist_credit_to_metadata()

artists_credits_from_node() now returns:
(artists, artists_string, artists_sort_string, artists_ids)

Modify test case according to this change.