Consider specifying a testing IDL for a mock payment UI

WebKit has converted some of the manual payment-request tests into automated tests by creating a mock payment UI in our testing harness that can be scripted from the test. Here's what the IDL looks like right now:

interface MockPaymentCoordinator {
    void setShippingAddress(MockPaymentAddress shippingAddress);
    void changeShippingOption(DOMString shippingOption);
    void changePaymentMethod(ApplePayPaymentMethod paymentMethod);
    void acceptPayment();
    void cancelPayment();

    readonly attribute ApplePayLineItem total;
    readonly attribute sequence<ApplePayLineItem> lineItems;
    readonly attribute sequence<MockPaymentError> errors;
};

This is specific to Apple Pay right now, and is missing some stuff, so obviously this wouldn't be used as-is.

But maybe we could specify something like this so that (a) the WebKit-specific tests could be upstreamed to WPT, and (b) other browsers could run them.

Start by reviewing the MockPaymentCoordinator IDL shown in the issue and the existing WebKit automated payment-request tests described there. Determine the scope of a browser-neutral testing IDL and what would be needed for the tests to be upstreamed to WPT and run by other browsers; done means the proposal is specified and agreed.