tests: move test/helpers to test/test_helpers.

The Rails convention is for test/helpers to be used for testing the view
helpers in app/helpers. We were using it to store certain utility
methods instead. Move these to test/test_helpers so that test/helpers
can be used for its intended purpose.

test/unit/moderator/tag_batch_change_test.rb

@@ -1,10 +1,7 @@
 require "test_helper"
-require 'helpers/saved_search_test_helper'
 
 module Moderator
   class TagBatchChangeTest < ActiveSupport::TestCase
-    include SavedSearchTestHelper
-
     def setup
       super
       mock_saved_search_service!