KSY schema: validate it against current tests
We've got our https://github.com/kaitai-io/ksy_schema — it is just one file in a repo now.

Shall we introduce some testing / validation for the schema itself? Likely the most straightforward way would be running [all existings test formats](https://github.com/kaitai-io/kaitai_struct_tests/tree/master/formats) against the current schema. All these formats should both work in ksc + pass validation against the schema.

Cc @dgelessus @generalmimon @fudgepop01?