Add tests for factories and compat

I think this is intended for use in tests that need to run on multiple versions. Something like:
```python
def test_fields(self):
    expected_fields = {
        'id',
        'foo',
        'foo_id',
    }
    expected_fields = wipe_id_fields_on_django_lt_17(expected_fields)

    fields = Model._meta.get_all_field_names()
    self.assertCountEqual(fields, expected_fields)
```