Instead of relying on black-box content, how about make an ``initialize()`` that sets up the conditions you expect, then verify them in ``run_once()``?

There are many ways to do this, but what I would do is:

*  invent a random string with ``utils.generate_random_string(12)``, store it inside stuff
*  Start up a container from the default image with command ``/bin/bash -c "cat - > /path/to/file"``
*  Pass the random string as ``stdin`` into ``execute()``.

Now in ``run_once()`` you can run your ``"cp /path/to/file %s" % self.tmpdir``, open that file up and compare it's contents to the random string stored in stuff.  If anything breaks along the way, it tosses an exception and the test fails (as it should).