Validate schema against known JSON documents

[aslakhellesoy]

In order to validate the schema itself, we should verify that it validates all the JSON documents in https://github.com/cucumber/cucumber-json-testdata-generator

Because these JSON documents are generated by different implementations and versions of Cucumber, it may be necessary to maintain multiple versions of the JSON Schema (for multiple combinations of Cucumber implementation and version).