public async void ProcessMessages_Successful()
        {
            // Arrange
            MessageSystem   messageSystem = new MessageSystem();
            UnitTestMessage message       = new UnitTestMessage();

            messageSystem.RegisterReceiver <UnitTestMessage>(HandleMessage);
            messageSystem.SendMessage(message);

            // Act
            await messageSystem.ProcessMessagesAsync();

            // Assert
            Assert.Empty(messageSystem.ActiveMessages);
            Assert.True(callbackExecuted);
        }