FedCM "core": break Web Platform tests between "core" and "extensions"

[samuelgoto]

Along with #703, we would like to use Web Platform tests to define us define concrete how we are defining "core" and "interoperable implementation" (e.g. passing tests).

Currently, the wpt/fedcm web platform tests covers features beyond "core", so we'd like to move "extensions" into their own directory, so that we can make it clear what we would expect 2 independent implementations to pass.

Something that we have discussed was to also use the WebDriver APIs commands defined here to help us achieve the interoperation between the browser engines.

One interesting discussion that I think we left without a conclusion was rather to use the WebDriver API for IdPs to use as regression tests (e.g. in selenium tests) or if we should keep that as purely an API for browser engineers to achieve interoperation. It wasn't clear what the answer was, but it seemed like an important observation that currently they are being used for both things.

[wseltzer]

Discussed in 11 March meeting.