benchmark: don't check wrk in non-http benchmark

When running a non-http benchmark, there is no need the check for the
wrk tool so move the wrk check into the http method.

PR-URL: https://github.com/iojs/io.js/pull/1368


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>