code: def signal_stop(self, test_id=None):
      
      if test_id is None:
        test_id = self._default_test_id
      if self._analyses[test_id].ts_end:
        return CONSTANTS.OK
      self._analyses[test_id].ts_end = naarad.utils.get_standardized_timestamp('now', None)
      return CONSTANTS.OK
docs: Set ts_end for the analysis represented by test_id
      :param test_id: integer that represents the analysis
      :return: test_id