test: remove `common.PORT` from gc tests

Allow the operating system to provide an arbitrary available port rather
than using `common.PORT`, as `common.PORT` makes it likely that a test
will fail with `EADDRINUSE` as a side effect of an earlier test.

PR-URL: https://github.com/nodejs/node/pull/7013


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>