[specs] add test for discriminated models with union discriminator

Please add a test for a scenario like this

```tsp
@service
namespace Test;

@discriminator("type")
model ResponseFormat {
type: "text" | "json_object" | "json_schema";
}

model ResponseFormatText extends ResponseFormat {
type: "text";
}

op foo(): ResponseFormatText;
```

You can choose clearer names and test more scenarios