def expected_checksum(self, test_name):
        """Returns the checksum of the image we expect the test to produce,
        or None if it is a text-only test.
        """
        png_path = self.expected_filename(test_name, '.png')

        if self._filesystem.exists(png_path):
            with self._filesystem.open_binary_file_for_reading(
                    png_path) as filehandle:
                return read_checksum_from_png.read_checksum(filehandle)

        return None