Fix flaky localization tests (#1664)

Fixes #1663

We shouldn't replace the whole Rails' app's RouteSet, particularly when the `@routes`
instance variable already provides the hook.

Instead we can mirror what Rails does internally:
https://github.com/rails/rails/blob/a59ddc714fa0403cbd8c7ae6abdec2f71ee7b63f/actionpack/test/controller/test_case_test.rb#L1224C1-L1236C8

test/controllers/application/localization_test.rb

@@ -3,20 +3,16 @@
 class ApplicationControllerTest < ActionController::TestCase
   include Devise::Test::ControllerHelpers
 
-  def setup
-    ApplicationController.class_eval do
-      def any_action
+  setup do
+    @controller = Class.new(ApplicationController) do
+      def locale
         render plain: I18n.locale
       end
-    end
-
-    # If disable_clear_and_finalize is set to true, Rails will not clear other
-    # routes when calling again the draw method. Look at the source code at:
-    # https://www.rubydoc.info/gems/actionpack/ActionDispatch/Routing/RouteSet#draw-instance_method
-    Rails.application.routes.disable_clear_and_finalize = true
+    end.new
 
-    Rails.application.routes.draw do
-      get "any_action" => "application#any_action"
+    @routes = ActionDispatch::Routing::RouteSet.new
+    @routes.draw do
+      get "locale" => "anonymous#locale"
     end
 
     @actual_locales = I18n.available_locales
@@ -32,23 +28,23 @@ def teardown
   test "team locale is nil, user locale is nil" do
     sign_in @user
 
-    get :any_action
+    get :locale
     assert_equal I18n.default_locale.to_s, response.body
   end
 
   test "team locale is nil, user locale is nil, HTTP_ACCEPT_LANGUAGE equals es" do
-    @request.headers["HTTP_ACCEPT_LANGUAGE"] = "es"
+    request.headers["HTTP_ACCEPT_LANGUAGE"] = "es"
     sign_in @user
 
-    get :any_action
+    get :locale
     assert_equal "es", response.body
   end
 
   test "team locale is es, user locale is nil" do
     sign_in @user
     @user.current_team.update!(locale: "es")
 
-    get :any_action
+    get :locale
     assert_equal "es", response.body
   end
 
@@ -57,15 +53,15 @@ def teardown
     @user.current_team.update!(locale: "es")
     @user.update!(locale: "de")
 
-    get :any_action
+    get :locale
     assert_equal "de", response.body
   end
 
   test "team locale is nil, user locale is de" do
     sign_in @user
     @user.update!(locale: "de")
 
-    get :any_action
+    get :locale
     assert_equal "de", response.body
   end
 
@@ -74,26 +70,26 @@ def teardown
     @user.update!(locale: "")
     @user.current_team.update!(locale: "es")
 
-    get :any_action
+    get :locale
     assert_equal "es", response.body
   end
 
   test "user not signed in" do
-    get :any_action
+    get :locale
     assert_equal I18n.default_locale.to_s, response.body
   end
 
   test "user not signed in and browser sends HTTP_ACCEPT_LANGUAGE" do
-    @request.headers["HTTP_ACCEPT_LANGUAGE"] = "de"
+    request.headers["HTTP_ACCEPT_LANGUAGE"] = "de"
 
-    get :any_action
+    get :locale
     assert_equal "de", response.body
   end
 
   test "user not signed in and browser sends HTTP_ACCEPT_LANGUAGE with unknown value" do
-    @request.headers["HTTP_ACCEPT_LANGUAGE"] = "this-language-does-not-really-exist"
+    request.headers["HTTP_ACCEPT_LANGUAGE"] = "this-language-does-not-really-exist"
 
-    get :any_action
+    get :locale
     assert_equal I18n.default_locale.to_s, response.body
   end
 end