Don't include unneeded helpers in tests

Including them might lead to conflicts since two methods might have the
same name. For example, we're getting some exceptions when taking
screenshots of a failing test, because the method `image_path` from
`ActionView::Helpers::AssetUrlHelper` has the same name as a method used
to save the screenshot.

Besides, we were including all helpers in places were only the `dom_id`
method is used, and in other places where no helper methods were used at
all. So we can just invoke `ActionView::RecordIdentifier.dom_id`
directly.

spec/shared/features/nested_documentable.rb

@@ -1,8 +1,4 @@
 shared_examples "nested documentable" do |login_as_name, documentable_factory_name, path, documentable_path_arguments, fill_resource_method_name, submit_button, documentable_success_notice|
-  include ActionView::Helpers
-  include DocumentsHelper
-  include DocumentablesHelper
-
   let!(:administrator)          { create(:user) }
   let!(:user)                   { create(:user, :level_two) }
   let!(:arguments)              { {} }