benchmark: use strict mode

Apply strict mode to benchmark code.

PR-URL: https://github.com/nodejs/node/pull/5336


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>