Restore TestCase structure

[PhantomWatson]

• The conventional structure of /tests/TestCase that mirrored that of /src has been replaced with putting all unit tests into a single, flat directory. Avoiding the directory structure that reflects each class's namespace is going to make the codebase harder to manage and may interfere with autoloading.
• Speaking of autoloading, CommentariesTableTest.php has the namespace App\Test\TestCase\Controller, which doesn't reflect what kind of class it is or what directory it's in.