Need to add config.assets.compile=true to access `.assets`

sprockets-rails will only add `Rails.application.assets` if compile is
no false. See change on sprockets-rails
https://github.com/rails/sprockets-rails/commit/d7c7ee19991c565eb77ee143be2d009ba4472122

railties/test/application/assets_test.rb

@@ -191,6 +191,7 @@ class User < ActiveRecord::Base; raise 'should not be reached'; end
 
     test "asset pipeline should use a Sprockets::Index when config.assets.digest is true" do
       add_to_config "config.action_controller.perform_caching = false"
+      add_to_env_config "production", "config.assets.compile = true"
 
       ENV["RAILS_ENV"] = "production"
       require "#{app_path}/config/environment"