Immediately drop lock

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

tokio/tests/sync_mutex.rs

@@ -127,7 +127,7 @@ async fn aborted_future_2() {
     }
     // This should succeed as there is no lock left for the mutex.
     timeout(Duration::from_millis(1u64), async move {
-        let _g = m1.lock().await;
+        let _ = m1.lock().await;
     })
     .await
     .expect("Mutex is locked");