benchmark: add test double HTTP benchmarker

Refactor benchmark/_http-benchmarkers.js and add a test double
HTTP benchmarker for testing.

PR-URL: https://github.com/nodejs/node/pull/12121
Refs: https://github.com/nodejs/node/issues/12068


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>