FF |
  ...                                   REPORT_ONLY_FIRST_FAILURE)
  True

Only reporting flags can be set:

  >>> doctest.set_unittest_reportflags(ELLIPSIS)
  Traceback (most recent call last):
  ...
  ValueError: ('Only reporting flags allowed', 8)

  >>> doctest.set_unittest_reportflags(old) == (REPORT_NDIFF |
  ...                                   REPORT_ONLY_FIRST_FAILURE)
  True
z