Unify the JSON Schema Test Suite

The JSON Schema validation test suite currently maintains a separate folder for each released version of the specification. This means most tests exist as identical copies across multiple folders, and every new test or bug fix must be applied in several places. This does not scale. The annotation test suite already solves this problem using a `compatibility` field that indicates which versions each test applies to, allowing test runners to filter accordingly. This project brings that same approach to the validation test suite, collapsing all per-version folders into a single unified suite with no duplicate tests.