Test for STDERR from Qpsmtpd::Address->new(), and correct the failure

which was causing noisy tests.
Okay, so returning '<>' there is correct from the perspective of the tests. But it would blow up here on a null sender:

    $milter->send_rcpt_to($address->format));

Generally the result of any call to Address is an object, which generally evaluates to '<>' for a null sender (see "use overload").