'L{PLAINCredentials.setResponse} parses challenge strings of the
form::
NUL<authn-id>NUL<secret>'

def test_credentialsSetResponse(self):

    cred = imap4.PLAINCredentials()
    cred.setResponse('\x00testuser\x00secret')
    self.assertEqual(cred.username, 'testuser')
    self.assertEqual(cred.password, 'secret')
