i3: add test of keybinding merge logic

(cherry picked from commit c42206db02)

tests/default.nix

@@ -20,5 +20,7 @@ import nmt {
     "git/with-str-extra-config" = ./modules/programs/git-with-str-extra-config.nix;
     texlive-minimal = ./modules/programs/texlive-minimal.nix;
     xresources = ./modules/xresources.nix;
+  } // pkgs.lib.optionalAttrs pkgs.stdenv.hostPlatform.isLinux {
+    "i3/keybindings" = ./modules/services/window-managers/i3-keybindings.nix;
   };
 }

tests/modules/services/window-managers/i3-keybindings.nix

@@ -0,0 +1,34 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+  config = {
+    xsession.windowManager.i3 = {
+      enable = true;
+
+      config.keybindings =
+        let
+          modifier = config.xsession.windowManager.i3.config.modifier;
+        in
+          lib.mkOptionDefault {
+            "${modifier}+Left" = "overridden-command";
+            "${modifier}+Right" = null;
+            "${modifier}+Invented" = "invented-key-command";
+          };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/i3/config
+
+      assertFileRegex home-files/.config/i3/config \
+        'bindsym Mod1+Left overridden-command'
+
+      assertFileNotRegex home-files/.config/i3/config \
+        'Mod1+Right'
+
+      assertFileRegex home-files/.config/i3/config \
+        'bindsym Mod1+Invented invented-key-command'
+    '';
+  };
+}