Add tests for remove_backref.
Add tests for remove_backref.

 Also, add a test for dot, but the dot support we have is in-place
 for debugging. If this feature ends up breaking, it's likely better
 to rip the dot function out rather then fix the test.

opencivicdata/pupa,datamade/pupa,mileswwatkins/pupa,influence-usa/pupa,mileswwatkins/pupa,opencivicdata/pupa,rshorey/pupa,influence-usa/pupa,datamade/pupa,rshorey/pupa