Resetting is free if you discard the overlayfs writes, no? I am not sure if one can discard at runtime, or if the next test should be run in a new container. But that should still be fast.

If your db is small enough to fit in tmpfs, than sure, that is hard to beat. But then xfs and zfs are overkill too.

EDIT: I see you mentioning that starting the db is slow due to wiping and filling at runtime. But the idea of a snapshot is that you don't have to do that, unless I misunderstand you.