Document the metaschema URI used for draft-next tests for authors of new tests and implementers wanting to run them

The draft-next folder contains tests for in-development next versions of the specification.

It allows us to get ahead of testing before the spec is complete.

At the minute in order to run these tests there's at least one notable extra step:

Tests like this one are how we represent validating schemas against metaschemas. But the URL used there for draft-next is a 404. Of course someone wanting to run that test can inject a metaschema for that URL to their implementation's schema store. But ideally, if we do already publish metaschemas iteratively for the next version of the spec, we should use the URL those are published under, and somehow remember to update it when "freezing" the draft-next folder on release.

Start with tests/draft-next/defs.json and inspect the metaschema URL used there. Determine whether an iterative draft-next metaschema is published and identify the URI that authors and implementers should use. Done means documenting that URI and recording how it should be updated when draft-next is frozen for a release.