Fixed error with import

The import workaround didn't actually work.
Python does not handle "import x as y; from y import z"
and it worked only by accident.

qubesadmin/tests/app.py

@@ -27,9 +27,10 @@ import multiprocessing
 
 try:
     import unittest.mock as mock
+    from unittest.mock import call
 except ImportError:
     import mock
-from mock import call
+    from mock import call
 
 import tempfile