test: add test-npm-install to parallel tests suite

Currently we are not testing that `npm install` works.

This is a very naive / basic test that shells out to `npm install`
in an empty `tempDir`. While this test will not be able to check
that `npm install` is 100% working, it should catch certain edge
cases that break it.

PR-URL: https://github.com/nodejs/node/pull/5166


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Alexis Campailla <orangemocha@nodejs.org>