test: use random ports where possible

This helps to prevent issues where a failed test can keep a bound
socket open long enough to cause other tests to fail with EADDRINUSE
because the same port number is used.

PR-URL: https://github.com/nodejs/node/pull/7045


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>