Add support for testing unmanaged models

[ojake]

As per #264.
This is marker pytest.mark.django_use_model that allows testing of models that have Meta.managed = False

[ojake]

Ok, there is something weird going on. Looks like tests marked with django_use_model are not even executed. I discovered it when trying to use it with coverage.
I'm going to try to figure this out, but I would appreciate some help, because I clearly don't understand how pytest-django works.

Here's an example of "passing" test
https://github.com/ojake/pytest-django/blob/bug-no-test/tests/test_database.py#L190

[blueyed]

Let's keep the PR open..
I currently don't have time to look into it, but we'll come to it.

[ojake]

Thanks!

[timb07]

Hi. I don't understand py.test well enough to work out why
@pytest.mark.django_use_model(Unmanaged)
causes the test not to be collected, but I have found a way that does work.

Mark with:
@pytest.mark.django_use_model(model=Unmanaged)

And then in _django_use_model(), fetch the model(s) like this:
model = marker.kwargs['model']
instead of using request.getfuncargvalue().

[blueyed]

Closing this in favor of #310.