Rails engine testing controller using helper method of main app in rspec

I have one main rails app, and I have an engine within it, now the engine has a different test, I need to test one controller which is using main app MailerHelper module. How can I access in my spec.

My engine's user controller

module ControlPanel 
  class UserController < ApplicationController
    include MailerHelper
    def index 
      @users = User.all
    end
  end
end

I am using MailerHelper have many other methods which sent mails. As this is controller is in my engine control panel, following is specification in my engine

require 'rails_helper'

describe ControlPanel::UsersController, type: :controller do
  describe '#index' do
    it 'should display all users' do
      get :index
      expect(response).to render_template('index')
    end
  end
end

The test case is just simple, but as in my engines User controller I am using my main application helper, so it can't take it, I have seen many replies but nothing working, like including in rails_spec and ::MailerHelper etc.