MediaWiki uses a back-compat layer to preserve most of the PHPUnit 4
runtime, so existing tests will continue to run fine with minimal
modification. Once PHP < 7.0 support is dropped, we can drop PHPUnit 4
support, and adapt tests to use PHPUnit 6 features directly.
Bug: T177132
Depends-On: I884b240307e3fcad253aa64eeae9944ed4eb7d50
Change-Id: I497712c7693d137be197a69873baa58383ce9646
