test(test_login_logout): fix AssertionError
test(test_login_logout): fix AssertionError

Follow up to  c4fbf0c, two consecutive logouts does not raise
AssertionError anymore.

It was somewhat wrong to rely on this error anyway, because assert
statements are not reliable in Python (they are skipped when the
-O is passed to the interpreter.)

Change-Id: Ie4e99ef80b022c527865fd703b33dee4654b8ae9

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