tests: fix duplicated entries in TestVMsCollection

Deduplicate entries when iterating over TestVMsCollection values. Some
tests add given VM multiple times, to have it available under different
kind of keys (name, uuid etc) - similar to the real VMsCollection.

qubes/tests/vm/__init__.py

@@ -56,7 +56,7 @@ class TestVMsCollection(dict):
         self.clear()
 
     def __iter__(self):
-        return iter(self.values())
+        return iter(set(self.values()))
 
 class TestVolume(object):
     def __init__(self, pool):