tests: do not drop 'qubes.tests.' prefix

This cause troubles with "external" tests (loaded from outside of
qubes.tests module).

qubes/tests/__init__.py

@@ -223,7 +223,7 @@ class QubesTestCase(unittest.TestCase):
 
     def __str__(self):
         return '{}/{}/{}'.format(
-            '.'.join(self.__class__.__module__.split('.')[2:]),
+            self.__class__.__module__,
             self.__class__.__name__,
             self._testMethodName)