Create a public test plan

[rladuca]

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.