test: use ES6 in test-debugger-client.js

implements ES6 const and let instead var in test-debugger-client.js

PR-URL: https://github.com/nodejs/node/pull/10183


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>