[tests] test archivebot.DiscussuionPage.is_full() method
[tests] test archivebot.DiscussuionPage.is_full() method

- add tests for DiscussuionPage.is_full() method
- simplify TestDiscussionPageObject tests

Change-Id: Iaa9b82b32bc88043cfe6d445bf70b41e1e1a5cc6

wikimedia/pywikibot-core,wikimedia/pywikibot-core