Add unit test scaffolding for XHPAST/PHPAST

Summary:
Ref T4334. Allows us to build tests against this so we can accomplish goals described in T4334 with greater confidence.

This allows three types of tests to be defined:

• pass: Input is expected to parse into a specific tree.
• fail-syntax: Input is expected to fail with a syntax error.
• fail-parse: Input is expected to parse the wrong tree.

The latter two test types allow us to write passing tests of broken/bad behavior while keeping it clear which tests are passing-and-good and which are passing-but-broken.

Test Plan: Ran unit tests.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4334

Differential Revision: https://secure.phabricator.com/D8025