Add test plan to differential.revision.search

Summary: Ref T13151. Ref PHI622.

Test Plan: Loaded a revision in the Conduit UI; observed presence of testPlan field.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T13151

Differential Revision: https://secure.phabricator.com/D19518