Bisect forker prep refactorings

Created by: myronmarston

I've been working on a substantial performance improvement to --bisect that works by swapping out Bisect::Runner (now called Bisect::ShellRunner as of this PR) with a new runner that uses fork { } instead of shelling out so we can pay the cost of booting RSpec and loading any --require files one time, instead of paying it for each subset spec run.

The new Bisect::ForkRunner will be coming in a later PR. This PR just does a bunch of prepatory refactoring that is needed for the new runner.