Remove article xfail and added LoginPage

bug https://bugzilla.mozilla.org/show_bug.cgi?id=910149 is now fixed
Added LoginPage because when signing out we are redirected to https://support.allizom.org/en-US/users/auth
and it will be helpful for issue https://github.com/mozilla/sumo-tests/issues/227
<code>register_page</code> was removed in pull https://github.com/mozilla/sumo-tests/pull/217, but the page still exists