Addition of activejob testing guide to testing.md [ci skip]

We need to test two things:

1. Jobs themselves: how they behave when they're performed
2. Controllers, models, and other code that enqueue jobs

For 1. we shouldn't need anything special. Just create the job, perform it, and make assertions.

For 2. we need to use a test queue adapter so we can inspect which jobs are enqueued and make assertions about them.