Add JavaScript integration tests

Note: this is a __work in progress__.

Last remaining issue (probably): we're hung up on the fact that the local drush server is on a specific port (8080), but the frame loader / origin validation logic doesn't take ports into account.

We can either __a)__ add support to the loader to account for ports in its event origin validation logic or __b)__ attempt to force the drush server to load over standard port 80 (e.g. `sudo php /path/to/drush rs 127.0.0.1:80 &`)

Made several attempts at the latter, but ran into issues where the server would always respond with a 500. Difficult to reproduce locally and thus, difficult to debug. It may be appropriate to add port support instead.