test: listen on and connect to 127.0.0.1

Avoid transient DNS issues in test sequential/test-net-GH-5504 by using
the IP address instead of the 'localhost' host name.

Fixes: https://github.com/nodejs/node/issues/6611
PR-URL: https://github.com/nodejs/node/pull/7524


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>