Add an `id` property to all test cases

[Julian]

Add an `id` property to all test cases

[mwadams]

If you'd like to assign me to this, I'll take it on.

I propose the following in response to this:

1. Use UUIDs for the IDs, and make them globally unique, rather than just unique within the version of the spec.
2. Using Array index for the tests themselves is extremely brittle - people keep arbitrarily adding tests in the middle of the lists, and it is difficult to enforce. I propose using a UUID for each test within the test suite.
3. I will write a tool to update the IDs for everything and land a complete update all in one go
4. If a test changes in any way, I propose that its ID should be changed, and we should document that in the README
5. Note that with respect to (4) a test that relies on the metaschema should not be considered to be changed if the metaschema changes.

[Relequestual]

I originally presumed we would have some sort of human readable ID. UUIDs would be simpler in some respects.
I guess it depends on the intended use case.

If someone has a list of tests they skip, having human readable names which partially suggest the sort of test in question, it could be helpful. However, this wouldn't be the best if we agree that IDs should change if the test changes (which I think is a good idea).

In that case, I wonder if a simpler approach is to just MD5 the test object... although I suspect because of JSON and object ordering, that might not end up the same depending on various things.

All that to say, are we convinced UUIDs as IDs would be the best here?

[mwadams]

I did consider an MD5 hash of the test object (and the schema in the case of the test case) but that makes it rather harder to generate while developing the tests, and feels like a barrier to entry.

[Julian]

I don't really have any strong preference on the structure of the IDs -- I think I mentioned I think slugs are better because of the extra benefit (of being user readable and usable for other reasons in isolation) but if I'm not doing the work I definitely don't care.

The only thing I do care about is:

I will write a tool to update the IDs for everything and land a complete update all in one go

I don't personally want to (ever) review large automated PRs (and have said elsewhere that as far as I have seen in "real life" it's simply impossible for any human to do.) -- meaning I'm happy to review the script you write to do this, or else for someone else who thinks it's possible to do the review -- though experience in this repo itself has reaffirmed as far as I'm concerned that this is the case :) -- the last two times I said this (no large automated PRs) and others reviewed it, the PRs indeed introduced subtle test bugs that were fixed over a few months afterwards (not that it took months to fix, it took months to notice, which is often the problem).

So yeah, happy to review the script, or else to say "find any other test suite contributor willing to review the PR".

Otherwise I'm fine with UUIDs if you prefer.