inline test runner check into `as/testing/autorun.rb`.

This makes it possible to easily get the runner working with existing
setups that rely on `active_support/testing/autorun.rb`.

activesupport/lib/active_support/testing/autorun.rb

@@ -2,4 +2,11 @@
 
 require 'minitest'
 
-Minitest.autorun
+if Minitest.respond_to?(:run_with_rails_extension)
+  unless Minitest.run_with_rails_extension
+    Minitest.run_with_autorun = true
+    Minitest.autorun
+  end
+else
+  Minitest.autorun
+end

railties/lib/rails/test_help.rb

@@ -9,10 +9,7 @@
 require 'action_dispatch/testing/integration'
 require 'rails/generators/test_case'
 
-unless Minitest.run_with_rails_extension
-  Minitest.run_with_autorun = true
-  require 'active_support/testing/autorun'
-end
+require 'active_support/testing/autorun'
 
 if defined?(ActiveRecord::Base)
   ActiveRecord::Migration.maintain_test_schema!