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