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