Convert other exercises that have assertions to test mode

exercises/threads/threads3.rs

@@ -48,6 +48,7 @@ fn send_tx(q: Queue, tx: mpsc::Sender<u32>) -> () {
     });
 }
 
+#[test]
 fn main() {
     let (tx, rx) = mpsc::channel();
     let queue = Queue::new();