definitions.json should be renamed

[awwright]

"definitions.json" suggests it'll be about the keyword "definitions" instead of the meta-schema.

Perhaps we should move this file, and meta-schema tests from ref.json, into its own file under optional?

[Julian]

+1, but I think we should address #223 first

[Julian]

Over time I think we've continued to put meta schema tests alongside the same file and I guess it's worked OK. Perhaps at some point we should formally restructure things a bit, but will maybe close this for now. These tests are not optional definitely, they rely on properly implementing all the validators used in the meta-schema, which are of course the same ones required for implementation.