tools: print stderr on bad test.py `vmArch` check

This makes it so you can see why the check fails if it does.

Typically that sort of thing can happen if you are modifying
bootstrapping or `process`.

PR-URL: https://github.com/nodejs/node/pull/6786


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>