Add schema validation tests to schemacode

[tsalo]

I'd like to add a few tests to schemacode to ensure that the schema is "valid". We should probably define rules for the schema here to ensure that they make sense.

One test should check that all objects referred to in the rules portion of the schema are actually defined in the objects portion of the schema.

[tsalo]

Another test could check that the fields in each object definition are all allowed based on the object type, per #1048.