Tests: Add a check to validate required properties

[duncandewhurst]

Since RDLS makes fairly extensive use of required properties, we're often finding that they are not kept in sync with changes to property names so it would be helpful to include this in the schema tests. Otherwise, there's a risk of the schema requiring users to add undocumented properties in order for their data to be valid.

I think the best place to implement the check is as a part of the JSCC library that we're using for testing so I've opened an issue there: open-contracting/jscc#12