test: swap var for let/const throughout

Swap var for let/const throughout the common.js module.
Change a snake case variable to camel case starting on line 168.

PR-URL: https://github.com/nodejs/node/pull/10177


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>