Returns a callback to convert test record to proto and save to disk.

def save_to_disk(self, filename_pattern=None):
    """Returns a callback to convert test record to proto and save to disk."""
    if not self._converter:
      raise RuntimeError(
          'Must set _converter on subclass or via set_converter before calling '
          'save_to_disk.')

    pattern = filename_pattern or self._default_filename_pattern
    if not pattern:
      raise RuntimeError(
          'Must specify provide a filename_pattern or set a '
          '_default_filename_pattern on subclass.')

    def save_to_disk_callback(test_record_obj):
      proto = self._convert(test_record_obj)
      output_to_file = callbacks.OutputToFile(pattern)

      with output_to_file.open_output_file(test_record_obj) as outfile:
        outfile.write(proto.SerializeToString())

    return save_to_disk_callback