Fixes #2526.

This reverts commit 9f74ef94e7. The related bug was fixed in Django 1.11.4

Also remove some Django 1.6 compatibility code.

Fixes #2176.

This was removed from the parent class way back in Django 1.4, and was
not being used at all here. It existed only for backwards compatibility.

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.