Require only minitest.

minitest/autorun required minitest/spec and we are avoiding to require
it.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -3,7 +3,7 @@ require 'uri'
 require 'active_support/core_ext/kernel/singleton_class'
 require 'active_support/core_ext/object/try'
 require 'rack/test'
-require 'minitest/autorun'
+require 'minitest'
 
 module ActionDispatch
   module Integration #:nodoc:

activesupport/lib/active_support/test_case.rb

@@ -1,5 +1,5 @@
 gem 'minitest' # make sure we get the gem, not stdlib
-require 'minitest/autorun'
+require 'minitest'
 require 'active_support/testing/tagged_logging'
 require 'active_support/testing/setup_and_teardown'
 require 'active_support/testing/assertions'