[ActiveJob] Add queue_name_prefix

I think you're missing what the suggestion is about. It doesn't matter what the default value is right now, this test shouldn't care. The default value could change later, and then this test would be leaking state. The code should look like this:

```ruby
test do
  original_queue_name_prefix = ActiveJob::Base.queue_name_prefix
  original_queue_name = HelloJob.queue_name

  ActiveJob::Base.queue_name_prefix = 'aj'
  HelloJob.queue_as :low
  assert_equal 'aj_low', HelloJob.queue_name
ensure
  ActiveJob::Base.queue_name_prefix = original_queue_name_prefix
  HelloJob.queue_name = original_queue_name
end
```