tests: fix missing unittest.mock

The unittest.mock module needs to be imported before use. It worked by
coincidence for most unit tests, which do import it, but not for
integration tests which mostly dont.

Fixes: 151010d "Validate default_kernel global property on set"

qubes/tests/__init__.py

@@ -45,6 +45,7 @@
 import time
 import traceback
 import unittest
+import unittest.mock
 from distutils import spawn
 
 import gc