Add characterization test for group role assignment listing

There was no test that shows what happens when a group assignment is
created and the role assignments are listed (for all backends). The
LDAP assignment backend works incorrectly in that it returns user
assignments for group assignments.

Change-Id: I69bc3501cc5eae3039d805ed006821d6927f0440
Partial-Bug: #1365787
Merge "Add characterization test for group role assignment listing"