Another step toward removing unittest.TestCase.

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

We don't have a way to do multi-assert in the pytest we're running, so
cobble it together ourselves.

I guess the original line was wrong, but it would have been nice for
unittest2pytest to fix it for me:
https://github.com/pytest-dev/unittest2pytest/issues/52

One step of moving to pure pytest tests.

The conditionals are now getting unwieldy, perhaps we can simplify them
in the future?