Move models to respective test modules

What needs to be sorted out:

1. What to do with models that are referenced in multiple test modules. Cloning them into each test module feels bad because of <i>unDRYing</i> the code massively.

2. Same as the 1st, but relating to <a href="https://github.com/tomchristie/django-rest-framework/blob/b106ebd2c0a19107f12d5b87cfbe0083aaaa60b9/tests/models.py#L17">RESTFrameworkModel</a>, since multiple models use it from all over the place, and the only thing it does is set `app_label`, maybe remove it completely and set app label specifically on each model them from a common constant `TESTS_APP_LABEL='tests'` somewhere in settings i.e. `conftest.py` or `utils.py`.

<b>P. S.</b> As a side question. why don't asserts in tests set `response.data` as the message arg i.e. to `assertEqual()` so that when it fails the developer can see the error message, not just the assertion failure message?