simplify ActiveSupport.test_order definition

Based on feedback from https://github.com/rails/rails/pull/18077/files#r22069210
There is no need to initialize with `nil`, [it is already initialized in case it does not exist](https://github.com/rails/rails/blob/08754f12e65a9ec79633a605e986d0f1ffa4b251/activesupport/lib/active_support/core_ext/module/attribute_accessors.rb#L58).