Fully-qualify reference to Swift's Actor protocol in macro expansion code for synchronous test functions

[stmontgomery]

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:

// 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 #747, which first landed in Swift 6.1 and Xcode 16.3.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test