Add test for `config.active_storage.routes_prefix`

Follow up #33883.

railties/test/application/configuration_test.rb

@@ -2053,6 +2053,24 @@ module ApplicationTests
       assert_equal [ "password", "credit_card_number" ].to_set, ActiveRecord::Base.filter_attributes
     end
 
+    test "ActiveStorage.routes_prefix can be configured via config.active_storage.routes_prefix" do
+      app_file "config/environments/development.rb", <<-RUBY
+        Rails.application.configure do
+          config.active_storage.routes_prefix = '/files'
+        end
+      RUBY
+
+      output = rails("routes", "-g", "active_storage")
+      assert_equal <<~MESSAGE, output
+                           Prefix Verb URI Pattern                                                               Controller#Action
+               rails_service_blob GET  /files/blobs/:signed_id/*filename(.:format)                               active_storage/blobs#show
+        rails_blob_representation GET  /files/representations/:signed_blob_id/:variation_key/*filename(.:format) active_storage/representations#show
+               rails_disk_service GET  /files/disk/:encoded_key/*filename(.:format)                              active_storage/disk#show
+        update_rails_disk_service PUT  /files/disk/:encoded_token(.:format)                                      active_storage/disk#update
+             rails_direct_uploads POST /files/direct_uploads(.:format)                                           active_storage/direct_uploads#create
+      MESSAGE
+    end
+
     private
       def force_lazy_load_hooks
         yield # Tasty clarifying sugar, homie! We only need to reference a constant to load it.