If we have conditional fields, we often set `additionalProperties` `true`
at the top-level, but then we have to make sure to cover all the possible
combinations in conditionals, so unintended additions don't appear.
This revealed missing fields in delinvoice, for example.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This adds our first (basic) schema, and sews support into pyln-testing
so it will load schemas for any method for doc/schemas/{method}.schema.json.
All JSON responses in a test run are checked against the schema (if any).
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
