We should specifically check that a received email containing a

    X-DKIM-Authentication-Results: dunno

header doesn't successfully pass the domain checks called in
SMTPAutoresponder.runChecks().

 * CHANGE test_SMTPAutoresponder_runChecks_badDKIM() into
   test_SMTPAutoresponder_runChecks_DKIM_dunno() and
   test_SMTPAutoresponder_runChecks_DKIM_bad().