This makes testing/debugging easier by allowing to login as root
directly. The previous test system configuration added a step to this
process by forcing devs to login as test and use sudo to escalate to
root.
Having the declaration of system.stateVersion just adds more
maintainance for devs. For example, changing the nixpkgs value in
flake.nix requires a change of system.stateVersion in tests/default.nix.
awkawb