The test code now includes this:

    def test_shuffle_random_argument(self):
        # Test random argument to shuffle.
        shuffle = self.gen.shuffle
        mock_random = unittest.mock.Mock(return_value=0.5)
        seq = bytearray(b'abcdefghijk')
        shuffle(seq, mock_random)
        mock_random.assert_called_with()

Thank you for the suggestion.