Fix loop termination in TestQueuePacketConnWriteToKCP.

The noise-generating goroutine was meant to stop when the parent
function returned and closed the `done` channel. The `break` in the loop
was wrongly exiting only from the `select`, not from the `for`.

This was the cause of banchmark anomalies in
https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/issues/40260#note_2885832.
The noise-generating loop from the test was continuing to run while the
benchmarks were running.

common/turbotunnel/queuepacketconn_test.go

@@ -99,10 +99,11 @@ func TestQueuePacketConnWriteToKCP(t *testing.T) {
 		defer readyClose.Do(func() { close(ready) })
 		pconn := DiscardPacketConn{}
 		defer pconn.Close()
+	loop:
 		for {
 			select {
 			case <-done:
-				break
+				break loop
 			default:
 			}
 			// Create a new UDPSession, send once, then discard the