test: pass python path to node-gyp

node-gyp rebuild should use the same python interpreter as in Makefile
rather than let node-gyp guess the python path by itself.

PR-URL: https://github.com/nodejs/node/pull/6646


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