Add policy enforcement tests

This test does the following:

 * Replaces the PZP's policy with a policy denying access to everything except service discovery
 * Runs the PZP
 * Uses zombiejs to load a browser, pointing to a page attempting to use the get42() function
 * Returns 'success' if the page fails to use the API

Note that these tests are not integrated with Travis, yet, although they could very easily be.

Jira issue :WP-996