'The checker created by the C{--auth} command-line option returns a
L{Deferred} that returns the avatar id when presented with credentials
that are known to that checker.'

def test_authSuccess(self):

    self.options.parseOptions(['--auth', ('file:' + self.filename)])
    checker = self.options['credCheckers'][(-1)]
    correct = credentials.UsernamePassword(*self.usernamePassword)
    return checker.requestAvatarId(correct).addCallback((lambda username: self.assertEqual(username, correct.username)))
