Improve the messages for test assertion failures
    
    Use unittest.assertTrue with a message instead of assert statements in
    the tests. Rearrange the deprecated assert_ tests to use
    unittest.assertIn.
    
    Use unittest.skipIf instead of simply returning so that the test is
    counted as skipped instead of successful.
    
    Signed-off-by: Jan Hutar <jhutar@redhat.com>