This would allow you for example to run integration tests on github actions, asside from it being much faster (since you don’t need to boot a vm!)
Running tests on GH actions is possible, but awfully slow. See https://github.com/wamserma/flake-programs-sqlite/blob/7c3620d465d5da7ca7fef92a24a72b0bc871506e/test.nix which requires 3m 44s.
What exactly do you mean by “userland virtualization”?
virtualization without having to boot a vm,
eg docker, bsd jails, “bubblewrap” or something similar.
It is inefficient to boot a vm and causes issues like kvm not being supported by github actions.
I think on hercules I ran into another issue such as a limit on how many virtual machines could run at the same time on qemu, this sorta bottle-necks the ci pipeline, because only 1 or 2 jobs could run at the same time.
Ah yes, then no.
In their current state NixOS tests rely quite heavily on VMs (AFAIK).