Pass correct filename into PHPUnitTestEngine coverage

Summary: The test result parser in PhpunitTestEngine was receiving $test_path from the previous loop instead of $path from the current one. The variable isn't actually used in the PhpunitResultParser object (it exists for strict compatibility with the parent class) so it didn't cause any problems, but who knows if that could change in the future

Test Plan: Review diff. No changes to the output of running 'arc unit' when using the Phpunit engine, as expected

Reviewers: epriestley

CC: aran, epriestley, aurelijus, chad

Differential Revision: https://secure.phabricator.com/D6587