Isolation tests intentionally avoid loading any state (because they're often testing things that have their own load path semantics that should not be polluted), so rack/test is not yet on the load path. Moving require "rack/test" into the setup means and after boot_rails means that it'll be required after the laod path has been altered to add in the Rails vendor/gems

railties/test/application/routing_test.rb

@@ -1,18 +1,18 @@
 require 'isolation/abstract_unit'
-require 'rack/test'
 
 module ApplicationTests
   class RoutingTest < Test::Unit::TestCase
     include ActiveSupport::Testing::Isolation
-    include Rack::Test::Methods
 
     def setup
       build_app
+      boot_rails
+      require 'rack/test'
+      extend Rack::Test::Methods
     end
 
     def app
       @app ||= begin
-        boot_rails
         require "#{app_path}/config/environment"
 
         Rails.application