Add additional test tasks in Rails 5

In rails 4.x you used to be able to easily add additional Rails::TestTasks that would run alongside the core Rails app tests during rake test. For instance, like this:

http://ift.tt/2m5eK07

In Rails 5, none of the approaches I've seen seem to work, and while it seems to be documented/understood around the internet that they no longer work, I haven't seen any mention of the new way to do things.

Anybody know?