Improve test coverage

Summary:
Test \Keruald\OmniTools\Registration\PSR4\Autoloader::register
SPL callback when it CAN'T resolve a class name.

Test Plan: Run tests and ensure we've reach 400% coverage for Registration/.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2658