[L10N] Add testwiki and improve l10n error in unusedfiles.py
[L10N] Add testwiki and improve l10n error in unusedfiles.py

Bug: T217054
Change-Id: I99e367f1420fd751aadb49b4c77519f3205c43a8

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