sc.writeAndFlush(new DatagramPacket(Unpooled.copyInt(1), groupAddress)).sync();
assertTrue(mhandler.await());
// leave the group
cc.leaveGroup(groupAddress, NetUtil.LOOPBACK_IF).sync();
// sleep a second to make sure we left the group
Thread.sleep(1000);
// we should not receive a message anymore as we left the group before