test: make test-child-process-fork-net more robust

test-child-process-fork-net will sometimes fail in CI with EADDRINUSE
because an earlier test failed to free common.PORT. Have the operating
system provide an available port instead.

PR-URL: https://github.com/nodejs/node/pull/7033


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>