tests: use dot to separate test name components

This way the name can be directly copied to cmdline to rerun that
particular test.

tests/__init__.py

@@ -129,7 +129,7 @@ class QubesTestCase(unittest.TestCase):
 
 
     def __str__(self):
-        return '{}/{}/{}'.format(
+        return '{}.{}.{}'.format(
             '.'.join(self.__class__.__module__.split('.')[2:]),
             self.__class__.__name__,
             self._testMethodName)