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