test: Introduce knowledge of FreeBSD jails

FreeBSD jails act differently than your average vm or similar
application container. All routing passes through one ip address,
which makes things like localhost or 0.0.0.0 resolve differently.

Introduce a helper that allows us to verify if we're in a jail
and another one for returning an ip address for localhost.

Also, skip one test instead of trading additional complexity
in common.js for one specific user scenario.

PR-URL: https://github.com/iojs/io.js/pull/1167


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>