Validate driver JSON schemas

[gilles-peskine-arm]

We're using JSON schemas to validate JSON driver descriptions. But who validates the schemas? We don't — but others do and found syntax errors in our schemas. Those particular errors apparently don't bother Python's jsonschema module (so far), but I don't know what it makes of the erroneous schema.

The goal of this issue is to validate the JSON schemas as part of a CI job (an all.sh check_xxx job, I suppose). As a validator validator validation test, the validation should fail on the version of the schemas from before #9674.