Fix failure with minitest 5.0.7

Require minitest/mock in test where stub method is needed.

Apparently this was being "imported" by minitest pride plugin, which
previously required "minitest/autorun", that required "minitest/mock",
making the method available by chance.

It has been changed in minitest:
595ce955c0

So we need to make sure we require what we really need where necessary.

actionview/test/template/url_helper_test.rb

@@ -1,5 +1,6 @@
 # encoding: utf-8
 require 'abstract_unit'
+require 'minitest/mock'
 
 class UrlHelperTest < ActiveSupport::TestCase