Split unittest for SMTPAutoresponder DKIM runChecks().

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().
Add _createMessage() utility to test_email_dkim.py.
Add unittest for checkDKIM() with `X-DKIM-Authentication-Results: dunno`.

 * ADD new unittest test_checkDKIM_dunno() to test_email_dkim.py which checks
   that a ``X-DKIM-Authentication-Results: dunno`` header properly returns
   False.
Add unittest for checkDKIM() with two `X-DKIM-Authentication-Results:` headers.

 * ADD a new unittest, test_checkDKIM_good_dunno(), to
   test_email_dkim.py, which checks that a received email with two
   ``X-DKIM-Authentication-Results:`` headers (the first one good, the
   second ``dunno``) properly returns False.

 * FIXES #12091. Actually, it doesn't. It just proves that the changes
   in my `fix/9874-automate-email-tests` branch fix #12091. Either way,
   it's fixed, these unittests prove it.
Merge branch 'fix/12091-dkim-dunno' into fix/9874-automate-email-tests
Add timeout to SMTPAutoresponder unittests.

Some of them were taking a *really* long time on some hosts,
particularly these two:

   - test_SMTPAutoresponder_reply_transport_invalid
   - test_SMTPAutoresponder_reply_transport_valid