[IMPR] use a list for tests in test_valid
[IMPR] use a list for tests in test_valid

- use extended_title_tests list for all remaining tests
- use subTest for every test in this list

Change-Id: I75399c0b0e6d95b4a7cafd23d07cdbf3f959e763

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