Require ActionView::Assertions in ActionController test_case.rb.

actionpack/lib/action_controller/test_case.rb

@@ -2,6 +2,7 @@
 require 'active_support/core_ext/object/to_query'
 require 'active_support/core_ext/module/anonymous'
 require 'active_support/core_ext/hash/keys'
+require 'action_view/testing/assertions'
 
 module ActionController
   module TemplateAssertions