Add test to check for required fields missing from properties

[duncandewhurst]

e.g. in JSON schema the following schema is valid, but in RDLS, if a property is listed in required, it should be defined in properties.

{
   "type": "object",
   "properties": {
      "a": {}
   },
   "required": ["b"]
}

The test would catch a couple of recent errors I made in PRs.