Bring back the helpers tests

Also keep the hook as other tools may rely on it,
we just don't do anything by default on Rails.

railties/lib/rails/generators/rails/helper/helper_generator.rb

@@ -6,6 +6,8 @@ class HelperGenerator < NamedBase # :nodoc:
       def create_helper_files
         template 'helper.rb', File.join('app/helpers', class_path, "#{file_name}_helper.rb")
       end
+
+      hook_for :test_framework
     end
   end
 end

railties/lib/rails/generators/test_unit/helper/helper_generator.rb

+require 'rails/generators/test_unit'
+
+module TestUnit # :nodoc:
+  module Generators # :nodoc:
+    class HelperGenerator < Base # :nodoc:
+      # Rails does not generate anything here.
+    end
+  end
+end

railties/test/generators/helper_generator_test.rb

+require 'generators/generators_test_helper'
+require 'rails/generators/rails/helper/helper_generator'
+
+ObjectHelper = Class.new
+AnotherObjectHelperTest = Class.new
+
+class HelperGeneratorTest < Rails::Generators::TestCase
+  include GeneratorsTestHelper
+  arguments %w(admin)
+
+  def test_helper_skeleton_is_created
+    run_generator
+    assert_file "app/helpers/admin_helper.rb", /module AdminHelper/
+  end
+
+  def test_check_class_collision
+    content = capture(:stderr){ run_generator ["object"] }
+    assert_match(/The name 'ObjectHelper' is either already used in your application or reserved/, content)
+  end
+
+  def test_namespaced_and_not_namespaced_helpers
+    run_generator ["products"]
+
+    # We have to require the generated helper to show the problem because
+    # the test helpers just check for generated files and contents but
+    # do not actually load them. But they have to be loaded (as in a real environment)
+    # to make the second generator run fail
+    require "#{destination_root}/app/helpers/products_helper"
+
+    assert_nothing_raised do
+      begin
+        run_generator ["admin::products"]
+      ensure
+        # cleanup
+        Object.send(:remove_const, :ProductsHelper)
+      end
+    end
+  end
+end