For testing purposes I usually put some logs or debugger into my code, probably as everybody does. To test my bot I would like to do the same but is there even a way to test/debug the messenger bot locally, or do I always have to deploy my tests?
Just fill the webhook url with the url that you use for testing. You can send something to bot, and bot can receive the messages(if you are the administrator of the app). from the received messages you could get your sender_id, you can use the sender_id as the recipient and let bot send messages to the recipient. In this step you can just send a POST request independently for testing. After finishing all the testing above, remove the debugging part, it's time to go to app review!