Force RAILS_ENV=development in rackup_test.rb. This fixes a problem on the CI server. For some reason the RAILS_ENV on the CI server is production, which means that the ActionDispatch::Static middleware is not used, and so the request in 'config.ru can be racked up' returns a 404 rather than a 200.

railties/test/application/rackup_test.rb

@@ -11,10 +11,16 @@ module ApplicationTests
     end
 
     def setup
+      @prev_rails_env = ENV['RAILS_ENV']
+      ENV['RAILS_ENV'] = 'development'
       build_app
       boot_rails
     end
 
+    def teardown
+      ENV['RAILS_ENV'] = @prev_rails_env
+    end
+
     test "rails app is present" do
       assert File.exist?(app_path("config"))
     end