Fix _not_ setting CLI options in tests
    
    Previously, we used mock.MagicMock objects which produced new mock
    objects for all accessed attributes. An argparse.Namespace object
    however (the result of parsing a command line) will have unset option
    attributes set to None, the new MainArgs class emulates this behavior.
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>