Undefined method test_helper, Rails 5

I added test/helpers/test_helper.rb file (which wasn't there by default) with this content and the method is_logged_in?:

ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'

# use minitest report
require 'minitest/reporters'
Minitest::Reporters.use!

class ActiveSupport::TestCase

  fixtures :all

  # Add more helper methods to be used by all tests here...
  def is_logged_in?
    !session[:user_id].nil?
  end
end

, but when I use it in integration test - test/integrations/user_signup_test.rb:

require 'test_helper'

class UsersSignupTest < ActionDispatch::IntegrationTest
  test "valid signup" do
    ....
    assert is_logged_in?
    ....
  end
end

and run rake test, it says :

UsersSignupTest#test_valid_signup: NoMethodError: undefined method is_logged_in?' for #<UsersSignupTest:0x000000060becc0> test/integration/users_signup_test.rb:30:in block in <class:UsersSignupTest>'