Push down test selection and add framework for output capture

This clearly is a WIP. We know have the reporter granularity to capture
per mutation test outputs + per mutation test selections.

This adds all infrastructure to address:

* #185 we know which tests are run in reporter
* #180 fine grained information available for the reporter
* #178 minitest project integration only needs to return an enumerable
  with metadata
* #174 We can now "see" all test, allowing to generate a default
  matcher.
* #158 Ability to steer test selection centralized for all integrations
* #125 We have the required objects in graph
* #96 We have finer granularity in reporter graph

Because we know signal more complex state from killforks to parent I
need to bring back killfork partent signalling, but this time with
sending complex data around (Test::Report). Should be easy with
Marshal.{dump,load} but my OSS time budget is exhausted for now.