test: move the debugger tests back to parallel

Run the debugger with `--port=common.PORT` to avoid the use of the same
port.

PR-URL: https://github.com/nodejs/node/pull/6246


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>