Add valid/invalid tests for discriminators

[postatum]

https://github.com/raml-org/raml-spec/blob/master/versions/raml-10/raml-10.md/#object-type

• The value must match the name of one of the declared properties of a type
• Unsupported practices are inline type declarations ...
• ... and using discriminator with non-scalar properties
• Neither discriminator nor discriminatorValue can be defined for any inline type declarations or union types
• discriminatorValue: Requires including a discriminator facet in the type declaration.