Monkey-patch ActionView::TestCase via on_load hook

Because Action View is not always in use when running Rails tests

Author: amatsuda

lib/test/unit/rails/test_help.rb

@@ -109,7 +109,9 @@ class << klass
   end
 end
 
-ActionView::TestCase.singleton_class.prepend TestUnitRails::ActionViewSubTestCase
+ActiveSupport.on_load :action_view_test_case do
+  ActionView::TestCase.singleton_class.prepend TestUnitRails::ActionViewSubTestCase
+end
 
 class ActionDispatch::IntegrationTest
   setup do