Rename PHPUnitTestEngineTestCase for consistency
Summary: Ref T5655. PHPUnitTestEngineTestCase is the test case for PhpunitTestEngine.
Test Plan: N/A
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: aurelijus, Korvin, epriestley
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D11203