test: introduce a broadcast helper
Introduce a helper - very similar to common.localhostIPv4
that allows test runner to override a broadcast address. This is required in certain scenarios - like FreeBSD jails - since networking acts a bit different.
Refs: https://github.com/nodejs/node/issues/2472
(I've enabled BROADCAST=$foo on both FreeBSD bots)
/R=@trott, ?