Streamline the test scripts

[rcantin-w]

What does this change?

Gareth's work here, where he rightly reused existing helpers made me realise that we were repeating some logic and defaults IDs at various points. I wanted to streamline it, make it potentially easier to add new types to (although when will that ever happen 😄 ). Would recommend reviewing without whitespace.

• Streamlines the test scripts to have one source of truth for default IDs
• Fixes the allowedtypes type, which wasn't working, we could enter whatever value and it would work

I do think it helps with readability and maintainability as well.

How to test

Run some tests scripts, see if it's expected results!
Run update-prismic-snapshot to see if it still works as expected.

How can we measure success?

Typescript is actually useful and we can add types/modify the tests/default IDs easily and in one place.

Have we considered potential risks?

This is mostly for testing purposes, but it also affected update-prismic-snapshot, so there could be that, although the only change is a check if the type isn't valid. I re-ran the snapshots locally and it looks alright to me.