unittest_mixins would check that files got created if a test made a
temporary directory, so that we could trim down making temp dirs
needlessly.

But we don't use unittest_mixins any more, so this setting does nothing.
Remove it.

We don't need to use the one from unittest_mixins.

The auto-decorating metaclass was interfering with parameterized methods
on Python 2.7.  But we don't need it anymore anyway, since pytest will
let us hook to deal with the exception in a simpler way.

Another step toward removing unittest.TestCase.

This is another step toward removing unittest.TestCase as a base class.