Add a section about job testing

Hello,

This pull request is just a follow up to #16921 addressing the different comments. This part makes the distinction between normal job testing and asserting that a job get enqueued/performed inside other components like models.

Also it makes the assumption that `ActiveJob::TestHelper`'s methods are available directly from `ActiveSupport::TestCase` as it should normally be the case (c.f. #17475).

Cross-refs #16576.

Have a nice day.
I think "different levels inside your application" is enough, we don't need the examples.