The auth method tests simply weren't adequate because of the fact that
smtpd doesn't support authentication.  I borrowed some of Milan's
code for that from issue #21935 and added it to the smtplib tests.
Also discovered that the direct test for the 'auth' method wasn't actually
testing anything and fixed it.

The fix makes the new authobject mechanism work the way it is
documented...the problem was that wasn't checking for a 334 return code
if an initial-response was provided, which works fine for auth plain
and cram-md5, but not for auth login.