[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`.
Related: https://github.com/padraic/mockery/issues/376, https://github.com/sebastianbergmann/phpunit/issues/1440.