Set policy for LLM-generated tests

[jgraham]

CC @web-platform-tests/wpt-core-team

I was recently asked about the policy for using LLMs to generate tests that are submitted to wpt. Currently we don't have any explicit policy on this, but I think we need one, so it's clear to test authors what's permissible, and to downstream consumers whether wpt is in line with any other policies they might have with regard to LLMs.

To be clear, I expect that any policy here would not affect review requirements i.e. we'd still require human review for all tests to ensure they're correct and easy to follow.