Add XHPAST test for nested quotes

Summary: Ref T1941. This result is wrong, but cover it with a test before I muck around with the parser itself (see D8029 for example).

Test Plan: Ran unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Maniphest Tasks: T1941

Differential Revision: https://secure.phabricator.com/D13962