Avoid a potential race condition and use-after-free when calling Test.cancel(). (#1395)

If a test creates an unstructured, non-detached task that continues
running after the test has finished and eventually calls
`Test.cancel()`, then it may be able to see a reference to the test's
(or test case's) task after it has been destroyed by the Swift runtime.

This PR ensures that the infrastructure under `Test.cancel()` clears its
reference to the test's task before returning. This then minimizes the
risk of observing the task after it has been destroyed.

Further work at the Swift runtime level may be required to completely
eliminate this race condition, but this change makes it sufficiently
narrow that any example I can come up with is contrived.

### 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/Test+Cancellation.swift

@@ -87,10 +87,19 @@ extension TestCancellable {
   /// the current task, test, or test case is cancelled, it records a
   /// corresponding cancellation event.
   func withCancellationHandling<R>(_ body: () async throws -> R) async rethrows -> R {
+    let taskReference = _TaskReference()
     var currentTaskReferences = _currentTaskReferences
-    currentTaskReferences[ObjectIdentifier(Self.self)] = _TaskReference()
+    currentTaskReferences[ObjectIdentifier(Self.self)] = taskReference
     return try await $_currentTaskReferences.withValue(currentTaskReferences) {
-      try await withTaskCancellationHandler {
+      // Before returning, explicitly clear the stored task. This minimizes
+      // the potential race condition that can occur if test code creates an
+      // unstructured task and calls `Test.cancel()` in it after the test body
+      // has finished.
+      defer {
+        _ = taskReference.takeUnsafeCurrentTask()
+      }
+
+      return try await withTaskCancellationHandler {
         try await body()
       } onCancel: {
         // The current task was cancelled, so cancel the test case or test