Add port_check_remote() & use it in smtp fixture

py.test -k test_email with firewall up & down to see the difference.

Currently, it fails with "TimedOutError: Could not do function <lambda>() in time" which is quite ambiguous.

@jkrocil is going to fix this and the function above, ``if port in _ports`` should always fail as the ``addr`` is the key, not the ``port``.