I've just released a simple #ActivityPub debugging tool. It's hosted on Glitch so you don't have to worry about spinning up servers or SSL certs etc. You clone your own copy of the project, set it up, and you can create ActivityPub accounts that can send any raw JSON payload you specify to its followers. I use it for testing new and novel ActivityPub objects, and to test compatibility with messages sent from remote servers without needing to create an account on them.