We don't need unittest_mixins' module cleaner anymore.

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.

These are copied from unittest_mixins, and adapted to pytest.

Now that we don't inherit from TestCase, pytest can parametrize methods.

I thought I knew when this passed and when it failed. Now that our tests
are not TestCase's, pytest is enforcing the xfails.  This passes locally
on Mac, but fails in CI on Mac.  So skip it.