Add a sched_yeild in tocket lock test..

test/test_ticket_lock.c

@@ -18,6 +18,7 @@ static void *update_thread(void *arg)
     struct ThreadArg *targ = arg;
     for (i = 0; i < targ->rounds; i++) {
         uc_ticket_lock(&lock);
+        sched_yield();
         g_counter++;
         uc_ticket_unlock(&lock);
     }