set_unittest_reportflags

Sets the unittest option flags.

The old flag is returned so that a runner could restore the old
value if it wished to:

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

  >>> doctest._unittest_reportflags == (REPORT_NDIFF |
  ...                                   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