Rails + Engine + Testing: How do you write a test for a mounted Engine (Forem specifically)?

I am using Forem for a project, and I've added a couple actions to the built in PostsController under app/decorators/controllers/forem/posts_controller_decorator.rb. I want to write a test using Test::Unit for this controller, but I can't seem to figure out how to do so.

I tried something like:

require 'test_helper'

module Forem
  class PostsControllerTest < ActionController::TestCase

    test 'GET :my_route'
      get :my_route
      assert_response :success
    end

  end
end

but it doesn't seem to work.

Has anyone written tests for a similar engine or even Forem?