Test for collection artists only

Before adding an option to retrieve collection and non-collection
artists, test that the default behavior is to return permanent
collection artists only.
Add collection=all parameter to #list

The default behavior of the list method is to return only artists who
have works in the permanent collection. With the collection=all
parameter set, return all artists (e.g. include artists who have
loaned works in the database but no permanent collection works)
Merge branch 'non-collection-artists' into develop