'L{PLAINAuthenticator.challengeResponse} returns challenge strings of
the form::
NUL<authn-id>NUL<secret>'

def test_authenticatorChallengeResponse(self):

    username = 'testuser'
    secret = 'secret'
    chal = 'challenge'
    cAuth = imap4.PLAINAuthenticator(username)
    response = cAuth.challengeResponse(secret, chal)
    self.assertEqual(response, ((('\x00' + username) + '\x00') + secret))
