tools: refactor `make test-npm` into test-npm.sh

Extracts test-npm from Makefile and puts it in tools/test-npm.sh
Also improves test-npm to use a separate copy of deps/npm for testing.

PR-URL: https://github.com/iojs/io.js/pull/1662


Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Evan Lucas <evanlucas@me.com>