docs: def get_slowest_files(cls, num=10):
          """Returns the slowest num files.
          
          :param num: The maximum number of results to be returned
          :type num: int
          
          """
          cur = cls.get_cursor()
          q = (
              "SELECT file, SUM(elapsed) as sum_elapsed FROM {} GROUP BY file"
              " ORDER BY sum_elapsed DESC LIMIT {}"
          )
          cur.execute(q.format(cls.meta['table'], num))
          result = cur.fetchall()
          # Don't return the weird invalid row if no tests were run
          if not all([cls.is_valid_row(x) for x in result]):
              return []
          return result