Added test to explicitly demonstrate manually defined related_name usage works in select_subclasses going forwards.
Added test to explicitly demonstrate manually defined related_name
usage works in select_subclasses going forwards.

Also fixed fragility of 2 tests introduced in ae71f9080e3eb83b3d2a98294b28dd3ba711196b

carljm/django-model-utils,patrys/django-model-utils,timmygee/django-model-utils,patrys/django-model-utils,yeago/django-model-utils,carljm/django-model-utils,nemesisdesign/django-model-utils,nemesisdesign/django-model-utils,yeago/django-model-utils,timmygee/django-model-utils