test: use ES6 to update let & const

Also updating assertStrict and moving the assert required.

PR-URL: https://github.com/nodejs/node/pull/9917


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>