Consolidated some many_to_one tests.
Fixed #23550 -- Normalized get_queryset() of RelatedObjectDescriptor
and ReverseSingleRelatedObjectDescriptor so they actually return QuerySet
instances.

Also ensured that SingleRelatedObjectDescriptor.get_queryset() accounts
for use_for_related_fields=True.

This cleanup lays the groundwork for #23533.

Thanks Anssi Kääriäinen for the review.