Add testing-base. (#36)

Add a base 23.11 nixos system with Gnome, GDM
and nix-flatpak installed.

testing-base/README.md

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+# Testing base
+
+A base GDM + Gnome `nixos` system to experiment with `nix-flatpak`.
+
+Build a qemu virtual machine with
+```bash
+nix build .#nixosConfigurations.test-system-module.config.system.build.vm
+```
+
+start the system with
+```bash
+export QEMU_NET_OPTS="hostfwd=tcp::2221-:22"
+result/bin/run-nixos-vm
+```
+
+and ssh into the vm with
+```bash
+ssh -oUserKnownHostsFile=/dev/null -oStrictHostKeyChecking=no antani@localhost -p 2221
+```

testing-base/common.nix

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+# configuration.nix
+{ config, lib, pkgs, ... }: {
+  users.users.antani = {
+    isNormalUser = true;
+    home = "/home/antani";
+    extraGroups = [ "wheel" ]; # Enables `sudo` for the user.
+    password = "changeme"; # The password assigned if the user does not already exist.
+  };
+
+  virtualisation.vmVariant = {
+    # following configuration is added only when building VM with build-vm
+    virtualisation = {
+      memorySize = 1024; # Use 1024MiB memory.
+      cores = 2;
+      graphics = true; # Boot the vm in a window.
+      diskSize = 1000; # Virtual machine disk size in MB.
+    };
+  };
+
+  services.openssh = {
+    enable = true;
+    settings.PasswordAuthentication = true;
+  };
+
+  networking.firewall.allowedTCPPorts = [ 22 ];
+  environment.systemPackages = with pkgs; [
+    git
+    vim
+  ];
+  services.xserver.enable = true;
+
+  # Enable the GNOME Desktop Environment.
+  services.xserver.displayManager.gdm.enable = true;
+  services.xserver.desktopManager.gnome.enable = true;
+
+  # Required to install flatpak
+  xdg.portal = {
+    enable = true;
+    config = {
+      common = {
+        default = [
+          "gtk"
+        ];
+      };
+    };
+    extraPortals = with pkgs; [
+      xdg-desktop-portal-wlr
+      #      xdg-desktop-portal-kde
+      #      xdg-desktop-portal-gtk
+    ];
+  };
+
+  system.stateVersion = "23.11";
+}

testing-base/configuration.nix

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+# configuration.nix
+{ config, lib, pkgs, ... }: {
+  imports = [
+    ./common.nix
+  ];
+  # nix-flatpak setup
+  services.flatpak.enable = true;
+  services.flatpak.update.auto.enable = false;
+  services.flatpak.uninstallUnmanagedPackages = true;
+  services.flatpak.packages = [
+    # ...
+  ];
+}

testing-base/flake.nix

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+# flake.nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-23.11";
+    flatpaks.url = "../";
+  };
+  outputs = inputs@{ self, nixpkgs, flatpaks, ... }:
+    let
+      system = "x86_64-linux";
+    in
+    {
+      # hostname = test-system-module
+      nixosConfigurations.test-system-module = nixpkgs.lib.nixosSystem {
+        inherit system;
+        modules = [
+          flatpaks.nixosModules.nix-flatpak
+          ./configuration.nix
+        ];
+      };
+    };
+}