test: fix test runner arg regression

Append --node-args to existing list, don't overwrite arg list.

Fixes: https://github.com/nodejs/node/issues/5442
PR-URL: https://github.com/nodejs/node/pull/5446


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-by: Michael Dawson <michael_dawson@ca.ibm.com>