/usr/lib/ruby/vendor_ruby/action_dispatch/testing/ is in ruby-actionpack 2:4.2.6-1.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/dom.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/response.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/routing.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/selector.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/assertions/tag.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/integration.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/test_process.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/test_request.rb
/usr/lib/ruby/vendor_ruby/action_dispatch/testing/test_response.rb