rails: using application helper method in integration test

I'm writing integration tests for some controller and I need to mock current_user from this (another) controller:

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
  helper_method :current_user

  private

  def current_user
    @current_user ||= User.first
  end
end

I've tried to overwrite @current_user in test, but it didn't work.