fix(tuic): wait for both traffic counters in the relay E2E tests

The race job failed on TestServerUDPDatagramE2E with Up:0 Down:1300.
BytesUp is added on the sending goroutine after the relay Send returns,
while BytesDown is added on the response goroutine, so the mock echo can
be counted and delivered before the upload is. The test drained the
counters once right after the reply and assumed both were present.

Production is unaffected: deltas left for the next collection window are
still summed. The TCP E2E test made the same assumption, so both now
accumulate drained deltas until up and down reach the payload size.

internal/tuic/server_test.go

@@ -196,12 +196,30 @@ func testServerTCPConnectE2E(t *testing.T, controller string) {
 		t.Fatalf("expected active email [email protected], got %v", activeEmails)
 	}
 
-	deltas := server.CollectClientTraffic()
-	if len(deltas) == 0 {
-		t.Fatalf("expected traffic deltas, got none")
-	}
-	if deltas[0].Email != "[email protected]" || deltas[0].Up < int64(len(testMsg)) || deltas[0].Down < int64(len(testMsg)) {
-		t.Fatalf("unexpected traffic deltas: %+v", deltas[0])
+	waitForClientTraffic(t, server, "[email protected]", int64(len(testMsg)))
+}
+
+// waitForClientTraffic accumulates drained deltas because the up and down counters are
+// bumped on different relay goroutines, so the echo can arrive before the upload is counted.
+func waitForClientTraffic(t *testing.T, server *Server, email string, minBytes int64) {
+	t.Helper()
+	var up, down int64
+	deadline := time.Now().Add(4 * time.Second)
+	for {
+		for _, delta := range server.CollectClientTraffic() {
+			if delta.Email != email {
+				t.Fatalf("unexpected traffic delta for %q: %+v", delta.Email, delta)
+			}
+			up += delta.Up
+			down += delta.Down
+		}
+		if up >= minBytes && down >= minBytes {
+			return
+		}
+		if time.Now().After(deadline) {
+			t.Fatalf("traffic for %s = up %d, down %d; want both >= %d", email, up, down, minBytes)
+		}
+		time.Sleep(5 * time.Millisecond)
 	}
 }
 
@@ -356,13 +374,7 @@ func testServerUDPDatagramE2E(t *testing.T, controller string) {
 	}
 
 	// 4. Verify traffic
-	deltas := server.CollectClientTraffic()
-	if len(deltas) == 0 {
-		t.Fatalf("expected traffic deltas, got none")
-	}
-	if deltas[0].Email != "[email protected]" || deltas[0].Up < int64(len(udpMsg)) || deltas[0].Down < int64(len(udpMsg)) {
-		t.Fatalf("unexpected traffic deltas: %+v", deltas[0])
-	}
+	waitForClientTraffic(t, server, "[email protected]", int64(len(udpMsg)))
 }
 
 func TestServerUDPStreamE2E(t *testing.T) {