Provide guidance around using GenAI to author accessibility tests

[rahimabdi]

Discussed in today's Interop Accessibility meeting (Nov 4 2025) with @janewman @zcorpan @cookiecrook.

It would be helpful to provide test authors with guidance on using GenAI such as noting that a test has been written (or co-written) with the assistance of AI, avoiding authoring single tests against multiple specs, performing a self-review before requesting feedback, etc.

This could be added to Accessibility Interop Project Overview and Contribution Guidelines.