`@Test` doesn't work consistently on operator functions

This works:

```swift
prefix operator ^

@Test(arguments: [(0)])
prefix func ^(_ lhs: Int) {
  #expect(1 == 3)
}
```

But if you remove the `_` argument label or change it to an `infix` or `postfix` operator, you get a compiler error in the macro expansion. Either `@Test` should properly handle these cases or the macro should detect and reject attempts to use it on an operator.