The internals of the test framework are pretty heavily tied to Nix and NixOS, so a release independent of Nix(OS) isn't probably going to happen. With that said, the test infrastructure itself is exposed as library functions that any Nix user can use. And even though the test VMs itself will run NixOS, the host machine can be any Linux distro with Nix installed.
I think it would be really difficult to get such a good experience with something that isn't NixOS. You might look at things like:
- http://open.qa/ - TestKitchen
But I must recommend, indeed, taking a good hard took at NixOS! :)
PLEASE consider releasing some form of this tool, or at least documenting the snot out of it... it looks incredible. We'd really use this at work.