Add tests for file_watcher config on update

[related #24243]

railties/test/generators/app_generator_test.rb

@@ -215,6 +215,20 @@ def test_rails_update_set_the_cookie_serializer_to_marshal_if_it_is_not_already_
     end
   end
 
+  def test_rails_update_dont_set_file_watcher
+    app_root = File.join(destination_root, 'myapp')
+    run_generator [app_root]
+
+    stub_rails_application(app_root) do
+      generator = Rails::Generators::AppGenerator.new ["rails"], [], destination_root: app_root, shell: @shell
+      generator.send(:app_const)
+      quietly { generator.send(:update_config_files) }
+      assert_file "#{app_root}/config/environments/development.rb" do |content|
+        assert_match(/# config.file_watcher/, content)
+      end
+    end
+  end
+
   def test_rails_update_does_not_create_active_record_belongs_to_required_by_default
     app_root = File.join(destination_root, 'myapp')
     run_generator [app_root]