Add a test to ensure tasks are balanced across workers

tokio-util/tests/spawn_pinned.rs

@@ -180,7 +180,7 @@ async fn tasks_are_balanced() {
     let thread_id1 = join_handle1.await.unwrap();
     let thread_id2 = join_handle2.await.unwrap();
 
-    println!("{:?} {:?}", thread_id1, thread_id2);
-
+    // Since the first task was active when the second task spawned, they should
+    // be on separate workers/threads.
     assert_ne!(thread_id1, thread_id2);
 }