[tests] Remove allowed_failure decorators
[tests] Remove allowed_failure decorators

- both test methods test_many_continuations_limited
  and test_two_continuations_limited passes. The
  decorator can be removed then
  and Bug: T75461 can be closed
- remove disabled _test_two_continuations_limited_long_test
  test because it needs 20' to process.

Bug: T223030
Change-Id: Ifcb2b543df11850316d4003234cd803c29f8e625

wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core