Modify coalesce_tracks() in order to handle the case where subtracks
are defined inside an index track (as `sub_tracks` attribute),
reorganizing the if logic to avoid duplicated code.
Add a try..catch block enclosing the call to clean_tracklist, as a
measure for avoiding side effects (and reverting to just parsing the
raw_list if there are any problems).
Revise the regular expression used for extracting information from a
discogs position, making the subtrack_index only be matched if one of
the other groups is matched as well. Split the definition into several
lines in order to try to document it a bit more clearly.
Add unit tests for position parsing and non standard positions.
Fix the underscore on tox dependency.
Add a first skeleton for providing unit tests for the discogs plugin,
heavily inspired by test_mb, which adds a couple of method for creating
very simple and not comprehensive Bags that mimic discogs_client.Release
and discogs_client.Track.
Add a test for checking the problem of setting media for the Tracks.
