Update the test schema to Draft 2020, and fix a bug in it.

[Julian]

minItems was placed inside items in the root schema, meaning it was
not checking that each file contained at least one test case. Now it
does, and does so using draft 2020.

Also tidied a few descriptions, removed a "dead property" (id) which we
added in prep for adding IDs to test cases but can revisit if/when we
get to that.

[gregsdennis]

If we add an enum property to the test files, we could probably key off of that and enforce a specific metaschema here. (Idea for separate PR)

[Julian]

As in provide some other mechanism for indicating a metaschema test? We could, but each of these changes is delicate, since someone's existing test runner still may not be expecting that. Obviously we're going to need to do something like breaking our informal backwards compatibility anyhow to resolve the optional/ folder situation, so we could in theory do so for that too, but lemme know if I misunderstood the idea entirely.

[gregsdennis]

No, I mean actually validate the schemas in the test files. If the test file specifies what version schemas its for (because JSON Schema does see the file path) then we can use that to select a metaschema to validate the test schema against.

I was thinking something like the below, but then I realized the test file is an array, and you'd have to specify the version on every test... Might be more headache than it's worth now that I see it.

---

Leaving for posterity

In the test file

{
  "version": "2020-12",
   ... // the rest of the test file
}

In the above schema:

{
  ...
  "oneOf": [
    {
      "properties": {
        "version": { "const": "2020-12" },
        "schema": { "$ref": "<2020-12 metaschema>" }
    },
    ... // more for other versions
  ]
  ...
}

[Julian]

Thanks again for the review!