webtransport: fix flaky accept queue test

p2p/transport/webtransport/transport_test.go

@@ -470,17 +470,26 @@ func TestAcceptQueueFilledUp(t *testing.T) {
 		return cl.Dial(context.Background(), ln.Multiaddr(), serverID)
 	}
 
-	for i := 0; i < 16; i++ {
-		conn, err := newConn()
-		require.NoError(t, err)
-		defer conn.Close()
+	const num = 16 + 1 // one more than the accept queue capacity
+	errChan := make(chan error, num)
+	for i := 0; i < num; i++ {
+		go func() {
+			conn, err := newConn()
+			if err != nil {
+				errChan <- err
+				return
+			}
+			_, err = conn.AcceptStream()
+			errChan <- err
+		}()
 	}
 
-	conn, err := newConn()
-	if err == nil {
-		_, err = conn.AcceptStream()
-	}
-	require.Error(t, err)
+	require.Eventually(t, func() bool { return len(errChan) > 0 }, 500*time.Millisecond, 10*time.Millisecond, "expected an error to occur")
+	require.Never(t, func() bool { return len(errChan) > 1 }, 50*time.Millisecond, 10*time.Millisecond, "only expected one error to occur")
+
+	// test shutdown
+	require.NoError(t, ln.Close())
+	require.Eventually(t, func() bool { return len(errChan) == num }, 500*time.Millisecond, 10*time.Millisecond)
 }
 
 type reportingRcmgr struct {