Create a public test plan

We need a public test plan that includes the following:

- A roadmap for test porting
- DRT porting
- Feature test porting
- Engineering system work (Helix, etc)
- A description of the various test pieces in WPF and how they interact with the CI build
- DRTs and relation to CI
- Feature tests and relation to PR validation
- Guidance on porting and adding new test pieces in DRT or Feature Tests
- Guidance on utilizing the various tests in order to validate a PR
- A test matrix including OS and general system requirements

Not all of these pieces need to be fully described immediately, but we should at least have some semblance of a road map and contributing guide. Work like dotnet/wpf#138 should be unblocked ASAP.