tests: mpv: refactor stubs definition

I am about to introduce more tests making use of the stubs, let's
centralize them in one file.

tests/modules/programs/mpv/mpv-invalid-settings.nix

@@ -4,24 +4,14 @@
 }:
 
 {
+  imports = [ ./stubs.nix ];
+
   programs.mpv = {
     enable = true;
     package = pkgs.emptyDirectory;
     scripts = [ pkgs.mpvScript ];
   };
 
-  test.stubs = {
-    mpv = {
-      extraAttrs.override = { ... }: pkgs.emptyDirectory;
-    };
-
-    mpvScript = {
-      extraAttrs = {
-        scriptName = "something";
-      };
-    };
-  };
-
   test.asserts.assertions.expected = [
     ''The programs.mpv "package" option is mutually exclusive with "scripts" option.''
   ];

tests/modules/programs/mpv/stubs.nix

@@ -0,0 +1,40 @@
+{ lib, pkgs, ... }:
+
+{
+  test.stubs = {
+    mpv-unwrapped = {
+      name = "mpv-unwrapped";
+      outPath = null;
+      buildScript = ''
+        mkdir -p $out/bin $out/share/applications
+        echo "Name=mpv" > $out/share/applications/mpv.desktop
+
+        cp ${pkgs.writeShellScript "mpv" "exit 0"} $out/bin/mpv
+        chmod +x $out/bin/mpv
+      '';
+
+      extraAttrs = {
+        meta =
+          let
+            stub = "stub";
+          in
+          {
+            description = stub;
+            longDescription = stub;
+            homepage = stub;
+            mainProgram = stub;
+            license = [ stub ];
+            maintainers = [ stub ];
+            teams = [ stub ];
+            platforms = lib.platforms.all;
+          };
+      };
+    };
+
+    mpvScript = {
+      extraAttrs = {
+        scriptName = "mpvScript";
+      };
+    };
+  };
+}