[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`.
The mockery listener doesn't work, and will probably be removed in a later version of mockery.

In my personal projects, I actually have this tearDown method in a trait: https://github.com/GrahamCampbell/Laravel-TestBench/blob/v1.1.4/src/Traits/HelperTestCaseTrait.php#L57.