controller.rb

class ArticlesController < ApplicationController
  def index
    @articles = Article.all
  end
end

controller_test.rb

require 'test_helper'

class ArticlesControllerTest < ActionController::TestCase
  test "GET index responds successfully" do
    get :index
    assert_response :success, @response.body # passes
    assert_template 'index' # expecting <"new"> but rendering with <[]>
  end
end