Use fork of web-platform-tests

[alancutter]

This change redirects the web-platform-tests submodule to https://github.com/alancutter/web-platform-tests using the nativeShippingAPI branch.
The intention here is to get better coverage of our "web-animations" target by using versions of the tests that restrict themselves to the API surface implemented by the target and supporting native browsers. This will be achieved in future patches.

This change builds upon #466.

[suzyh]

Is your fork of wpt based on the same version that the polyfill repo was previously importing? In other words, is any of the change in the expectations file here due to the set of tests changing under you?

[suzyh]

How much of a maintenance burden will this new wpt fork be? Do you have a plan to periodically merge in new versions of the wpt tests into your fork and here? I guess that's something we haven't really thought about yet with this or the spec-compliance-testing tool I've been working on.

[suzyh]

Aside from it not being clear whether there are any changes due to the set of tests changing under you, LGTM.

[alancutter]

It's kind of a maintenance burden unfortunately, the alternatives are having next to no coverage for web-animations or maintain an additional testing polyfill that enables the use of animation.ready, effect.getComputedTiming() and other features unsupported in native browsers used by web-platform-tests/web-animations.