tests: use unittest2 if available

This fixes a test failure on RHEL 6.

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

tests/test_phase_base.py

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