add mutability test thing

system/default.nix

@@ -10,6 +10,7 @@ in
     ./profile
     ./programs
     ./sessions
+    ./testing
   ];
   config = {};
   options.nixfiles = {

system/testing/default.nix

@@ -0,0 +1,6 @@
+{...}:
+{
+  imports = [
+    ./mutability.nix
+  ];
+}

system/testing/mutability.nix

@@ -0,0 +1,22 @@
+{ lib, pkgs, config, ... }:
+let
+  cfg = config.nixfiles.testing.mutability;
+  file = pkgs.writeTextFile {
+    name = "test";
+    text = ''
+      meow!
+    '';
+  };
+in
+{
+  options.nixfiles.testing.mutability = {
+    enable = lib.mkEnableOption "mutability test";
+  };
+
+  config = lib.mkIf cfg.enable {
+    environment.etc.mutability-test = {
+      mode = "0644";
+      source = file;
+    };
+  };
+}