[cleanup] Simplify TestLink.test_valid()
[cleanup] Simplify TestLink.test_valid()

- use a title_tests lists to be tested
- calculate site only once

Change-Id: I4eb94ac69fdc48912dd7aac86a9670667f161ee4

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