Test Plans can now be executed

A Test Plan could already generate Test Cases from the doc. What it couldn't do was run them together as the plan itself. Now it can.

• Execute the plan directly: run a Test Plan with the Test Cases you've selected, the same way you run a Test Set.
• Plans get an owner: the same ownership model Test Sets just got, defaulting to the creator.
• Cases listed on the plan: see every Test Case that belongs to the plan, right where the plan lives.
• The document is unchanged: you author plans exactly the way you did before.

Write the plan, generate its Test Cases, and run the whole thing from one place, no separate Test Set required.

#Sprint29