tests/default.nix: removed user test, added password declaration for root user

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.

flake.nix

@@ -1,7 +1,7 @@
 {
   description = "An Invoice Ninja package and module.";
 
-  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11";
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
 
   outputs =
     { self, nixpkgs, nix, }:

tests/default.nix

@@ -6,14 +6,10 @@
     ../nixos-module/invoiceninja.nix
   ];
 
-  system.stateVersion = "25.11";
-
   nixpkgs.config.allowUnfree = true;
 
-  users.users.test = {
-    isNormalUser = true;
-    extraGroups = [ "wheel" ];
-    initialPassword = "test";
+  users.users.root = {
+    initialPassword = "toor";
   };
 
   services.invoiceninja = {