Add test cases for the Union for Discriminated types

[lirenhe]

Today, there is no cases for Discriminated types in https://github.com/Azure/cadl-ranch/tree/main/packages/cadl-ranch-specs/http/type/model/inheritance

Based on https://microsoft.github.io/typespec/standard-library/discriminated-types, we should add test cases for

@discriminator("kind")
union Pet {
  cat: Cat,
  dog: Dog,
}

model Cat {
  kind: "cat";
  meow: int32;
}

model Dog {
  kind: "dog";
  bark: string;
}