spawn <5 threads in RwLock concurrent_read_write test

Loom wasn't working otherwise.

tokio/src/sync/tests/loom_rwlock.rs

@@ -67,19 +67,15 @@ fn concurrent_read_write() {
             });
         });
 
-        let rwclone = rwlock.clone();
-        let t4 = thread::spawn(move || {
-            block_on(async {
-                let guard = rwclone.read_owned().await;
-                //at this state the value on the lock may either be 0, 5, or 10
-                assert!(*guard == 0 || *guard == 5 || *guard == 10);
-            });
-        });
+        {
+            let guard = block_on(rwlock.clone().read_owned());
+            //at this state the value on the lock may either be 0, 5, or 10
+            assert!(*guard == 0 || *guard == 5 || *guard == 10);
+        }
 
         t1.join().expect("thread 1 write should not panic");
         t2.join().expect("thread 2 write should not panic");
         t3.join().expect("thread 3 read should not panic");
-        t4.join().expect("thread 4 read should not panic");
 
         let guard = block_on(rwlock.read());
         //when all threads have finished the value on the lock should be 10