test: revert "remove churn() task from lifo_stealable" (#8114)

This reverts commit 6c03e03.

Author: Darksonn

tokio/tests/rt_threaded.rs

@@ -730,6 +730,18 @@ fn lifo_stealable() {
         .unwrap();
 
     rt.block_on(async {
+        // Keep the runtime busy so that the workers that might steal the
+        // blocked task don't all park themselves forever.
+        //
+        // Since this task will always be woken by whichever worker is holding
+        // the time driver, rather than a worker that's executing tasks, it
+        // shouldn't ever kick the victim task out of its worker's LIFO slot.
+        let churn = tokio::spawn(async move {
+            loop {
+                tokio::time::sleep(Duration::from_millis(4)).await;
+            }
+        });
+
         let victim_task_joined = tokio::spawn(async move {
             println!("[victim] task started");
             task_started_tx.send(()).unwrap();
@@ -779,6 +791,7 @@ fn lifo_stealable() {
         // Before possibly panicking, make sure that we wake up the blocker task
         // so that it doesn't stop the runtime from shutting down.
         block_thread_tx.send(()).unwrap();
+        churn.abort();
         result
             .expect("task in LIFO slot should complete within 30 seconds")
             .expect("task in LIFO slot should not panic");