test: fix flaky test-net-socket-local-address

test-net-socket-local-address had a race condition that resulted in
unreliability on FreeBSD and Windows. This changes fixes the issue.

Fixes: https://github.com/nodejs/node/issues/2475
PR-URL: https://github.com/nodejs/node/pull/4109


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>