undefined method `users' for Tests

I'm trying to write a simple test but my application is not detecting 'users' keyword when I'm using it in my Test functions:

post_controller_test.rb

require 'test_helper'

class PostsControllerTest < ActionController::TestCase
  # test "the truth" do
  #   assert true
  # end

  test "should fetch facebook post" do
    sign_in users(:one)
    get(:save_posts_from_facebook_page,{'id'=>'my_id'},{'access_token'=>Rails.application.secrets.fb_access_token})
    assert_response :success
  end
end 

test_helper.rb

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

class ActiveSupport::TestCase
    include Devise::TestHelpers
  # Add more helper methods to be used by all tests here...
end

users.yml

# Read about fixtures at http://ift.tt/1BqMnxt

one:
  email: hello@testing.com
  encrypted_password: <%= Devise.bcrypt(User,'password') %>
  links: 
  causes: 

two:
  email: MyString
  password: MyString
  links: 
  causes: 

output of rake/Error:

Run options: --seed 42684

# Running:

E

Finished in 0.010275s, 97.3279 runs/s, 0.0000 assertions/s.

  1) Error:
PostsControllerTest#test_should_fetch_facebook_post:
NoMethodError: undefined method `users' for #<PostsControllerTest:0x000001042152f0>
    test/controllers/posts_controller_test.rb:9:in `block in <class:PostsControllerTest>'

1 runs, 0 assertions, 0 failures, 1 errors, 0 skips