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