Wip mds sessions

In unittest, the loader/runner interfaces are public and extensible, so plumbing in a single class and executing it from another python function like this is straightforward, supported and safe.

py.test is built around its own command line runner, and doesn't advertise any of its python-level infrastructure as public, so trying to do this trick of invoking a test from within a running python environment would be a bit contorted.  All the magic test discovery etc from be more of a hinderance because we'd just be tricking it into finding this one single class.

At the point where tests like this were (perhaps, in the future) running out on their own somewhere (perhaps calling back to the teuthology bits remotely) using a friendly test runner like py.test probably starts to make sense.