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.
Not sure if this is the right place to add this. As mentioned in my comments above, this is not something that we expect people to turn off, so I don't think it makes sense to include it in a template. Seems like we can just do this automatically somewhere inside Active Job?