This requires changing the interface a bit to pass the directory
containing the test modules separately, because strangely the "unittest"
thingy cannot load the test modules by their absolute path when changing
the current working dir to where the core dump should happen.
So put the directory onto the PYTHONPATH to work around that;
arguably it's even nicer for clients this way.

Change-Id: I0d1d39f33d0c086c6c3014d2f9ba93f0a3dc0128