test: mark test flaky on FreeBSD

Mark test-net-socket-local-address flaky on FreeBSD. It times out with
some frequency on CI.

Ref: https://github.com/nodejs/node/issues/2475
PR-URL: https://github.com/nodejs/node/pull/4016


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>