test: add cluster inspector debug port test

This commit adds a test for the debug port value in cluster
workers using the inspector debugger.

Refs: https://github.com/nodejs/node/issues/8201
Refs: https://github.com/nodejs/node/pull/8386
Refs: https://github.com/nodejs/node/pull/8550
PR-URL: https://github.com/nodejs/node/pull/8958


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>