pytest: add schema support for JSON responses.

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>

contrib/pyln-client/pyln/client/lightning.py

@@ -349,6 +349,7 @@ class UnixDomainSocketRpc(object):
                     "enable": True
                 },
             })
+            # FIXME: Notification schema support?
             _, buf = self._readobj(sock, buf)
 
         request = {