Test instance for matching class hierarchy on get_from_identity

Test instance for matching class hierarchy on get_from_identity

Fixed issue where a lazyload that uses session-local "get" against a target
many-to-one relationship where an object with the correct primary key is
present, however it's an instance of a sibling class, does not correctly
return None as is the case when the lazy loader actually emits a load for
that row.

Fixes: #5210
Change-Id: I89f9946cfeba61d89a272435f76a5a082b1da30c