Changed the inheritance hierarchy:
1. Renamed `SystemTestsMixin` to `SystemTestCase`
2. `SystemTestCase` is a child of `QubesTestCase`
3. All classes extending the prior `SystemTestsMixin` now just extend `object`
Now failure to load external tests shows in which entry point the error
happened and a useful traceback. The traceback extends from the "try"
statement down to the actual error line, but it does not include the
frames above, ie. from the invocation to the load_tests routine. This is
a limitation of Python itself and usually not a problem.
