#18586 - GenericRelationsTests separated into several tests

This intends to fix the first test case reported in [Trac ticket 18586](https://code.djangoproject.com/ticket/18586) as a "too long" unit test to be separated into several, more specific ones.

In particular, this PR splits the old ``modeltests.generic_relations.GenericRelationsTests.test_generic_relations`` unit test into several different unit tests, each one related to a single feature of Django generic relations. It also adds a ``setUp`` method to the ``TestCase`` to define the common attributes.