Cleans up execute_job

A while back I submitted [a PR](https://github.com/mperham/sidekiq/pull/1938) that exposed a way to customize how a job was executed, in so much as how perform was actually called on the worker. I duplicated that in the sidekiq/testing, and have since determined that was a mistake and am rectifying that here.

In doing what I talk about in that original PR, I realized there was still some finagling to get it to behave correctly for the specs, and then I realized that it wasn't very unified and didn't push through to the actual functionality that I wanted to make sure was working -- after some back and forth (captured on https://github.com/mperham/sidekiq/pull/2097) @mperham decided he'd rather remove it.

Also cleans up the drain method in the test harness.
If we consider it a public api, we should call through to it from the test harness as my first pull request did. ;-P

Aside from that, we can leave it I guess, but then the tests won't reflect any real world changes that might be made within it.