Is your feature request related to a problem?

Coming from #336 we are adding a missing field to a response body.

What solution would you like?

1. Spec testing should warn that the response contains a field that is not present in the spec.
2. It should be possible to run the tester with a flag that turns the warning in (1) into an error.

I believe this is not possible with AJV today, opened ajv-validator/ajv#2453.