test: implemented es6 conventions

implemented arrow functions and changed var to const where
appropriate.

PR-URL: https://github.com/nodejs/node/pull/9669


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>