Run webapi verifier tests automatically

Run tests automatically by:
* Getting request headers from POST of results rather than navigating to a separate page.
* Using fxos_appgen to install hosted app instead of navigating to an install page.
* Using new fxos_appgen.launch_app to run apps automatically

I also fixed a small (but annoying) bug in reporting the results.

This can't be merged until the corresponding changes to fxos_appgen land, but just in case someone wants to get an early start on a review, here goes :)