Basically I was looking for lemmy and mbin versions of https://piefed.social/c/playground - as I set up my single user instance, it would be good to have a test magazine where I can make test posts and test comments without bothering the users of a real community.
!Test@fedia.io was exactly what I was looking for, thanks!
Creating a magazine requires having a local account on the instance, which is fine for fedia.io but more generally isn’t always an option…