Add PHPAST test for remaining language features
Summary: Ref T4334. Adds yield, foreach-list, literal-deref tests. All currently fail. Adds a heredoc terminal test.
Test Plan: Ran unit tests.
Reviewers: btrahan, Firehed
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T4334
Differential Revision: https://secure.phabricator.com/D8031