Rails - test_helper.rb showing errors when using methods in other tests

I'm working through the tutorial at Railstutorial.org and am in the middle of Chapter 8 where I'm trying to utilize a test_helper.rb file which is located in

/test/test_helper.rb.

I have several tests running, one of which is calling a method I've defined in the test_helper.rb file.

Full test_helper.rb code:

ENV['RAILS_ENV'] ||= 'test'
require File.expand_path('../../config/environment', __FILE__)
require 'rails/test_help'
require "minitest/reporters"
Minitest::Reporters.use!

class ActiveSupport::TestCase
  fixtures :all

  # Returns true if a test user is logged in
  def is_logged_in?
    !session[:user_id].nil?
  end

end

My actual test file is located in:

/test/integration/users_signup_test.rb

With partial code:

require 'test_helper'

class UsersSignupTest < ActionDispatch::IntegrationTest

  test "valid signup information" do
    get signup_path
    assert_difference 'User.count', 1 do
      post_via_redirect users_path, user: { name:  "Example User",
                                            email: "user@example.com",
                                            password:              "password",
                                            password_confirmation: "password" }
    end
    assert_template 'users/show'

    #relevant call to method
    assert is_logged_in?
  end  

end

If I include the method in the actual test file itself everything works fine, but if I try to run a full suite of tests (rake test) I get the following error message indicating it can't use my helper method is_logged_in?

NoMethodError: undefined method `is_logged_in?' for #<UsersSignupTest:0x000000075cd530>

I'm new to Rails, but is there a reason it can't find the method in my test_helper.rb file?

I should note that everything technically works fine in Prod, and no actual failures occur in the test, just an error.