Add missing change for #39862 to 5.1-stable.

git-svn-id: https://svn.redmine.org/redmine/branches/5.1-stable@22554 e93f8b46-1217-0410-a6f0-8f06a7374b81

test/integration/routing/plugins_test.rb

@@ -94,6 +94,8 @@ class RoutingPluginsTest < Redmine::RoutingTest
         version '0.0.1'
       end
 
+      Redmine::Acts::Attachable::ObjectTypeConstraint.register_object_type('plugin_articles')
+
       Pathname(__dir__).glob("app/**/*.rb").sort.each do |path|
         require path
       end