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

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

test, inspector - server now works with the port 0 and test leverages that.