Perform case-insensitive check for PHPUnit test finder to not return original file

Summary: PhpunitTestEngine incorrectly included some source files as tests when run on a case-insensive fiesystem. This fixes that behavior. It could introduce problems for case-sensive users, but it's extremely unlikely to be an issue in practice (who would put a file in both Tests/ and tests/ and expect different results for the two?)

Test Plan: arc unit before and after change on OS X. Stubs and autoloaders in .../tests/... directories are no longer picked up.

Reviewers: epriestley

Reviewed By: epriestley

CC: aurelijus, Korvin, aran

Differential Revision: https://secure.phabricator.com/D7048