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);
}