Track new API for WPT accessibility testing beyond role and label

[cookiecrook]

We may be able to write more extensive WPT accessibility tests based on the future outcome of this AOM discussion:

• Computing the Accessible Backing Node, or a similar test-only interface WICG/aom#197

[cookiecrook]

I don't think we need to keep this open, it's not actionable.

The following two suffice as trackers:

• Computing the Accessible Backing Node, or a similar test-only interface WICG/aom#197
• Investigation: Accessibility Tree Dump API #51

Even as we expand testability, those newly possible tests will be part of spec testing (issues in the spec tracker) or a future interop project (issues in a new tracker), but unlikely there is anything needed for this 2023 Interop Investigation.