Payment Request API testing update

As requested by zkoch on IRC at today's meeting here's a quick update
on our testing efforts for the Payment Request API.

This efforts started with the goal to extend the set of tests
available here[0]. But doing so without an implementation appeared to
be challenging, so I worked on a stub implementation injected into
pages as a Chrome Extension[1] and was able to make initial progress
on [0] with [2] with promising results:

http://i.imgur.com/OeSdfxS.png

Plan is to merge [2] and keep adding as many tests as possible with
[1] and without the need to have a stubbed payment app server. I think
we can go pretty far in terms of testing with that and then evaluate
how me move towards testing the payment request / payment apps
interface.

Hope that is useful to you guys!

-stan

[0] https://github.com/w3c/web-platform-tests/tree/master/payment-request
[1] https://github.com/stan-stripe/js-payment-request
[2] https://github.com/w3c/web-platform-tests/pull/4768