…n code for synchronous test functions (swiftlang#1067)

This fixes a compilation error in code expanded from the `@Test` macro
when it's attached to a synchronous (i.e. non-`async`) test function in
a context where there is a concrete type named `Actor`. For example, the
following code reproduces the error:

```swift
// In MyApp
public class Actor {}

// In test code
import Testing
import MyApp

// ❌ 'any' has no effect on concrete type 'Actor'
//  - 'isolated' parameter type 'Actor?' does not conform to 'Actor' or 'DistributedActor'
@test func example() /* No 'async' */ {}
```

The macro code includes an unqualified reference to a type by that name,
but it's intended to refer to the protocol in Swift's `_Concurrency`
module. The fix is to ensure the macro's reference to this protocol is
fully-qualified with a module name.

This was first reported on the Swift Forums in
https://forums.swift.org/t/error-isolated-parameter-type-actor-does-not-conform-to-actor-or-distributedactor/79190.
This bug was introduced in swiftlang#747, which first landed in Swift 6.1 and
Xcode 16.3.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.