test: pass env vars through to test-benchmark-http

Allows NODE_TEST_DIR to be set (necessary to avoid path length issues
with common.PIPE).

PR-URL: https://github.com/nodejs/node/pull/13390
Refs: https://github.com/nodejs/node/issues/12708#issuecomment-297847882


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>