test: skip test if in FreeBSD jail

Test test-net-socket-local-address is flaky in FreeBSD jail but robust
otherwise.

Fixes: https://github.com/nodejs/node/issues/2475
PR-URL: https://github.com/nodejs/node/pull/3995


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>