* Add failing ActiveJob exceptions test for "disable retry jitter"

Thanks to @kaspth for the starting point.

* Update ActiveJob retry jitter to correctly use zero value

* Simplify "disable retry jitter" test

We don't need to repeat this many times. Fewer is shorter.

* Refactor determine_delay with jitter

* Fix indentation

* Close the curtains and give JITTER_DEFAULT some privacy

* Use .zero? instead of == to check jitter value

* Add ActiveJob test for explicit zero jitter

Co-authored-by: Kasper Timm Hansen <hey@kaspth.com>
Co-authored-by: Cliff Pruitt <cliff.pruitt@cliffpruitt.com>