[4.2] Improved The Test Suite

This does 3 things:
* This makes phpunit know about all the assertions mockery is doing under the hood.
* This enables phpunit strict mode so we know about output buffers left open (for example, anlutro was using strict mode when he found this bug: #5560) and buggy tests.
* I've changed `addUncoveredFilesFromWhitelist` to the newer `processUncoveredFilesFromWhitelist`.
IMO, an abstract parent class would perfectly do the job. Plus we could add useful methods to it, as custom assertions, `isStringNotNull`, `isEquivalentArray` (kind of assertions that do not exist out of the box) and so on...