The docs explain the structure of an event, be it JSON success/error results returned by the API, or what a cheer looks like through a chat connection. From that you can construct your own test data and just send it directly to the functions in your app you’ve set up to handle those events. You could even go as far as creating your own websocket server for your app to connect to in place of Twitch and send your own cheer notifications through that to simulate the process of a cheer entirely!
Just because Twitch doesn’t provide you a way to test your code, doesn’t mean that it’s not possible to test your own code yourself.