Clean up tests

[moggers87]

Look at this and weep:

https://github.com/moggers87/django-sendfile2/blob/aed66c86f810115477247ae193803dc7cf46bb56/django_sendfile/tests.py#L52-L56

Also none of the tests use override_settings, which they should.

There's probably a whole host of things that are wrong with those tests that need fixing too!