tests: Use unittest2 consistently

Without importing this module, the tests are going to fail on Python
2.6.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_config.py

@@ -2,7 +2,10 @@
 # -*- coding: utf-8 -*-
 
 
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
 
 import os
 import sys