Refactor Resque::Worker#prune_dead_workers

I was looking through CodeClimate for bad methods and found this one so
I decided I could refactor it.

Testing it was the hardest part.

I moved a lot of what was in #prune_dead_workers into some helper
methods to split up responsibility.

I also the order of the arguments passed to `assert_equal` in the
ProcessCoordinator test because the error output was confusing.

I tried my best to adhere to the style of the code I saw, but I'm more than happy to make any changes.