Make tests compatible with both `runtests.py` and `manage.py test`

I suppose it would be nice if we can test crispy_forms with both `runtests.py` and `manage.py test`.

It allows us to test both
* crispy_forms for all template packs (with `runtests.py`)
* the way crispy_forms is integrated into custom Django project (with `manage.py test`)

The pull request fixes existing incompatibilities with `manage.py test` (#181 and #182).