Attempt to build XHPAST during unit tests
Summary: If we have an out-of-date XHPAST binary, attempt to rebuild XHPAST as a part of running the unit tests. If XHPAST is missing (or we cannot build it for whatever reason), just skip the unit tests as per the existing behavior.
Test Plan: Deleted src/parser/xhpast/bin/xhpast and ran arc unit -- src/parser/xhpast.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11516