tests: yet another fix for integration tests cleanup code

Before waiting for remaining tasks on event loop (including libvirt
events), make sure all destroyed objects are really destroyed. This is
especially important for libvirt connections, which gets cleaned up only
when appropriate destructor (__del__) register a cleanup callback and it
gets called by the loop.

qubes/tests/__init__.py

@@ -410,6 +410,9 @@ class QubesTestCase(unittest.TestCase):
 
         global libvirt_event_impl
 
+        # really destroy all objects that could have used loop and/or libvirt
+        gc.collect()
+
         # Check for unfinished libvirt business.
         if libvirt_event_impl is not None:
             try: