Work around SwiftSyntax-related warning in CI. (#183)

We are getting a new warning in CI:

```
.../swift-testing/Sources/TestingMacros/ConditionMacro.swift:46:13: warning: let '_sourceLocationLabel' is not concurrency-safe because it is not either conforming to 'Sendable' or isolated to a global actor; this is an error in Swift 6
private let _sourceLocationLabel = TokenSyntax.identifier("sourceLocation")
            ^
```

Work around it by using `private var ... {}` instead of `private let`. I'm assuming that creating a `TokenSyntax` is not prohibitively expensive, so the change should be safe.

@ahoppen confirms that this operation is reasonably cheap and that syntax nodes are not safely sendable in the 5.9 tag of swift-syntax, so the warning is technically valid. Although our code does not access these constants across concurrency domains, the compiler is (in theory) free to invoke our macro expansion code on multiple tasks concurrently.

Concurrency safety in swift-syntax is tracked with swiftlang/swift-syntax#2425.

Author: grynspan

Sources/TestingMacros/ConditionMacro.swift

@@ -43,11 +43,11 @@ private protocol _ConditionMacro: ExpressionMacro, Sendable {
 
 /// The token used as the label of the source location argument passed to
 /// `#expect()` and `#require()`.
-private let _sourceLocationLabel = TokenSyntax.identifier("sourceLocation")
+private var _sourceLocationLabel: TokenSyntax { .identifier("sourceLocation") }
 
 /// The token used as a mandatory label on any (first) trailing closure used
 /// with `#expect()` or `#require()`.
-private let _trailingClosureLabel = TokenSyntax.identifier("performing")
+private var _trailingClosureLabel: TokenSyntax { .identifier("performing") }
 
 extension _ConditionMacro {
   public static func expansion(