[w3c/webpayments] Adopt a "test as you commit" policy (#234)

An increasing number of groups are beginning to make web-platform-tests part of their day-to-day work, and more are considering it, for example in w3c/dap-charter#48. https://github.com/foolip/testing-in-standards/blob/master/policy.md has some additional context, and https://github.com/w3c/accelerometer/pull/29 is an example of the changes we've been landing.

I can see that the payment-requests repository already has something similar in place in https://github.com/w3c/payment-request/blob/gh-pages/test-plan.md; I was wondering if you'd be open to doing the same to the other repositories:
* payment-method-id and payment-method-basic-card seem to be tested mostly via [payment request](https://github.com/w3c/web-platform-tests/tree/master/payment-request). I was thinking of adding a section to their `CONTRIBUTING.md` pointing to payment-request's `test-plan.md`, but using the boilerplate text we've been adding to other specs should also be fine.
* payment-handler seems to be being tested separately in [w-p-t](https://github.com/w3c/web-platform-tests/tree/master/payment-handler), so adding the usual text to its `CONTRIBUTING.md` should be fine.

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