Add $projectRoot to PytestTestEngine
Add $projectRoot to PytestTestEngine

Summary:
Fixes T8912. Property `$project_root` was missing in `PytestTestEngine` class, resulting in
broken py.test wrapper. Also renaming the property so the linter is happy.

Test Plan: `arc unit --everything`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: kparal, epriestley, Korvin

Maniphest Tasks: T8912

Differential Revision: https://secure.phabricator.com/D13698

julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist,julkiewicz/arcanist