test: don't assume broadcast traffic is unfiltered

parallel/test-dgram-broadcast-multi-process assumes non-local broadcast
traffic is unfiltered, an assumption that fails with, for example, the
default Fedora firewall rules.  Use a loopback interface instead.

Fixes: https://github.com/iojs/io.js/issues/219
PR-URL: https://github.com/iojs/io.js/pull/220


Reviewed-By: Fedor Indutny <fedor@indutny.com>