Add Active Job's custom assertions to test_helper

Hello,

In order to make Active Job's custom assertions available from any kind of test, let's include the `ActiveJob::TestHelper` by default inside `ActiveSupport::TestCase` through test_helper since it is mainly an
application-related feature.

This has originally been requested [here](https://github.com/rails/rails/pull/16724#discussion_r16918263) but it turns out that this hasn't been addressed. I'm not sure whether this is the right place to put this statement but being able to access these assertions from `ActiveSupport::TestCase` seems more to be an application rather than a framework feature. 

Have a nice day.