test requirements: Update pytest-mock to 1.2

- Try to import mock first instead of unittest.mock. This gives the user
  flexibility to install a newer mock version from PyPI instead of using
  the one available in the Python distribution.
- mock.sentinel is now aliased as mocker.sentinel for convenience.

misc/requirements/requirements-tests.txt

@@ -21,7 +21,7 @@ pytest-catchlog==1.2.2
 pytest-cov==2.3.0
 pytest-faulthandler==1.3.0
 pytest-instafail==0.3.0
-pytest-mock==1.1
+pytest-mock==1.2
 pytest-qt==2.0.0
 pytest-repeat==0.3.0
 pytest-rerunfailures==2.0.0