Update the schema for test suite tests to Draft 2019-09

[Julian]

The current schema is draft 4.

A bit of thought is warranted, and perhaps this interacts with #223, since in theory downstream authors may not support Draft 2019-09 (or any future draft), while they still might care about the schema for tests.

Perhaps this means that the test schema should actually go inside each draft.

Comments welcome of course from anyone with a $0.02 to spare.

[Relequestual]

I would say the test schemas should go within each draft!
As painful as it might be, we should still provide tests for draft-4, without preventing nice new things.

We should consider creating a policy...
We still haven't got a good way to reach out to key draft-4 users...

[Julian]

Creating a policy is #223 :) but agreed!

[Relequestual]

I knew I saw the word policy somewhere, but lost it again =D

[Julian]

Closed in #565.