Ticket #16920 -- Added GenericRelation tests

First test ensures related_name may not be set with GenericRelation.
Second test ensures that two models with the same name, but living in a
different app, do not cause a related_name clash when both defining a
GenericRelation to the same related model.
factor these 3 conditionals so they don' t have to be repeated in `_check_reverse_query_clashes` and `_check_accessor_clashes`?