[ActiveJob] TestCase

@dhh @rafaelfranca @cristianbica 
Lots of API surface area here to do just a little. These aren't obviously "opposites" either. I wouldn't call `enqueue_jobs` to turn *off* performing jobs. It makes it sound like enqueueing is off by default and I have to call this to enable.

Suggest delegating the reader/writer to the queue_adapter directly.