test: fix flaky test-net-write-slow

Increase socket timeout so that there is enough time to reliably run the
test on FreeBSD.

Fixes: https://github.com/nodejs/node/issues/7516
PR-URL: https://github.com/nodejs/node/pull/7555


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>