Remove a workaround for varargs not compiling on WASI. (#1323)

This PR removes a workaround on WASI where we were avoiding a call to
`withVaList()` because it miscompiled there. That problem was fixed by
swiftlang/swift#84029. It was cherry-picked to
6.2, but does not appear to be fixed in 6.2 CI, so the change here only
applies to 6.3 onward.

### 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.

Author: grynspan

Sources/Testing/Events/TimeValue.swift

@@ -81,7 +81,7 @@ extension TimeValue: Codable {}
 
 extension TimeValue: CustomStringConvertible {
   var description: String {
-#if os(WASI)
+#if os(WASI) && compiler(<6.3)
     // BUG: https://github.com/swiftlang/swift/issues/72398
     return String(describing: Duration(self))
 #else