Run a subtask of the runner task group for each individual test even when running serially. (#578)

Right now, when tests are run serially, they're simply run in a
for-loop. This has the side effect of potentially building up a lot of
autorelease pool cruft (on platforms with such a concept) if those tests
never introduce any suspension points.

As well, this produces a subtle difference in task cancellation
propagation because calling `UnsafeCurrentTask.cancel()` in a serialized
test will cancel the entire task group, not just the test's own subtask.

Resolves rdar://130034310.

### 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/Running/Runner.swift

@@ -121,21 +121,17 @@ extension Runner {
     _ body: @Sendable @escaping (E) async throws -> Void
   ) async throws where E: Sendable {
     let isParallelizationEnabled = step?.action.isParallelizationEnabled ?? configuration.isParallelizationEnabled
-    if isParallelizationEnabled {
-      // Each element gets its own subtask to run in.
-      try await withThrowingTaskGroup(of: Void.self) { taskGroup in
-        for element in sequence {
-          _ = taskGroup.addTaskUnlessCancelled {
-            try await body(element)
-          }
-        }
-        try await taskGroup.waitForAll()
-      }
-    } else {
-      // No task group or subtasks required as parallelization is not enabled.
+    try await withThrowingTaskGroup(of: Void.self) { taskGroup in
       for element in sequence {
-        try Task.checkCancellation()
-        try await body(element)
+        // Each element gets its own subtask to run in.
+        _ = taskGroup.addTaskUnlessCancelled {
+          try await body(element)
+        }
+
+        // If not parallelizing, wait after each task.
+        if !isParallelizationEnabled {
+          try await taskGroup.waitForAll()
+        }
       }
     }
   }