fish: use dummy fish package in tests

tests/modules/programs/fish/functions.nix

@@ -30,6 +30,13 @@ in {
       };
     };
 
+    # Needed to avoid error with dummy fish package.
+    xdg.dataFile."fish/home-manager_generated_completions".source =
+      lib.mkForce (builtins.toFile "empty" "");
+
+    nixpkgs.overlays =
+      [ (self: super: { fish = pkgs.writeScriptBin "dummy" ""; }) ];
+
     nmt = {
       description =
         "if fish.function is set, check file exists and contents match";

tests/modules/programs/fish/no-functions.nix

@@ -1,4 +1,4 @@
-{ config, lib, ... }:
+{ config, lib, pkgs, ... }:
 
 with lib;
 
@@ -10,6 +10,13 @@ with lib;
       functions = { };
     };
 
+    # Needed to avoid error with dummy fish package.
+    xdg.dataFile."fish/home-manager_generated_completions".source =
+      lib.mkForce (builtins.toFile "empty" "");
+
+    nixpkgs.overlays =
+      [ (self: super: { fish = pkgs.writeScriptBin "dummy" ""; }) ];
+
     nmt = {
       description =
         "if fish.functions is blank, the functions folder should not exist.";

tests/modules/programs/fish/plugins.nix

@@ -46,6 +46,13 @@ in {
       }];
     };
 
+    # Needed to avoid error with dummy fish package.
+    xdg.dataFile."fish/home-manager_generated_completions".source =
+      lib.mkForce (builtins.toFile "empty" "");
+
+    nixpkgs.overlays =
+      [ (self: super: { fish = pkgs.writeScriptBin "dummy" ""; }) ];
+
     nmt = {
       description =
         "if fish.plugins set, check conf.d file exists and contents match";