[tests] remove allowed_failure/allowed_failure_if from site_tests.py
[tests] remove allowed_failure/allowed_failure_if from site_tests.py

- test_image_usage_in_redirects: no reason given why this test should
  not run at travis
- test_preload_langlinks_normal passes
- remove deactivated test _test_preload_langlinks_long because
  it needs 20' to pass and the test is already done in
  test_preload_langlinks_normal
- test_preload_template passes
- test_preload_templates_and_langlinks fails due to unknown reason;
  mark it with unittest.expectedFailure decorator

Bug: T223030
Change-Id: I32f8b18b87954589faa8a722f99e4d819ea47442

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