validate result data against output schema

[karenetheridge]

There is a formal JSON Schema in the specification that describes the expected structure of the result (validity, errors, annotations etc). We should be able to pass the evaluator's output through this schema and generate a test failure when the output fails to match.

This should be a configurable option, although it might be reasonable to have it be enabled by default.

Some fixes are still needed on the output schema before this can be implemented (at least by default).

[karenetheridge]

A first cut of output tests are now available, via json-schema-org/JSON-Schema-Test-Suite#619.