test: have inspector test pick an open port

This ensures that cctest can be ran concurrently with other instances of
cctest or while the node is ran with --inspect.

Ref: https://github.com/nodejs/node/issues/10858
PR-URL: https://github.com/nodejs/node/pull/10861


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>