Composer: add phpunit/phpunit as dependency

[klees]

This PR adds phpunit/phpunit as composer dependency.

Usage:

• We use it to run unit tests.
• ILIAS/Component uses its mocking framework to analyze component definitions during build phase.

Wrapped By:

• Not applicable, provides the binary and classes for test management.

Reasoning:

• Its the standard PHP testing framework.

Maintenance:

• Won't go away any time soon.

Links:

• Documentation: https://phpunit.de