Spec: Skills functional tests

[gabog]

The working draft can be found here: https://github.com/microsoft/botframework-sdk/blob/gabog/SkillTestSpecUpdates2/specs/testing/skills/SkillsFunctionalTesting.md

[cleemullins]

Discussed w/ @gabog and the spec is in progress, but not needed to land as part of R11 (no code deliverable). Pushing to R12.