[IMPR] use *tests_modules as sets for better lookups
[IMPR] use *tests_modules as sets for better lookups

- reorder imports to PEP8 proposal: standard library, third party
  library, local application/library
- define library_tests_modules, script_test_modules and
  disabled_test_modules as sets because they are looked up later
- update library_tests_modules, script_test_modules
- sort library_tests_modules and  script_test_modules items
- return a set for extra_test_modules
- use itertools.chain to keep library/extra/script test order
- use modules in collector as generator instead creating another list
- 'l10n' tests are done on pywikibot-i18n repository;
  i18n.messages_available check can be removed

Change-Id: Ie86747af7715bd29f5252c2eb354364adae8ebba

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