[TEST] Add tests for flow.Topic class
[TEST] Add tests for flow.Topic class

Page https://test.wikipedia.org/wiki/User:Framawiki/pwb_tests/flow_board
was created for these tests.

Bug: T60941
Change-Id: Idfbae75bd8fd2bbd342a769504b44fd3e3c2ed16

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