Run webapi verifier tests automatically

[dminor]

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 :)

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/1622

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