Validate JSON schemas with a meta-schema

[ItsDrike]

_{I originally made this suggestion in #1055, though that issue is primarily about fixing the tints schema, but it did get an approval as something that should be implemented, so this just tracks it under a separate issue}

I think that it would make sense to have the tests also validate that the schemas used are in fact proper JSON schemas, and follow the JSON schema meta-schema, to prevent typos within the schemas themselves, leading to them not being properly enforced.