Separate MediaWiki unit and integration tests

Separate MediaWiki unit and integration tests

This changeset implements T89432 and related tickets and is based on exploration
done at the Prague Hackathon. The goal is to identify tests in MediaWiki core
that can be run without having to install & configure MediaWiki and its dependencies,
and provide a way to execute these tests via the standard phpunit entry point,
allowing for faster development and integration with existing tooling like IDEs.

The initial set of tests that met these criteria were identified using the work Amir did in
I88822667693d9e00ac3d4639c87bc24e5083e5e8. These tests were then moved into a new subdirectory
under phpunit/ and organized into a separate test suite. The environment for this suite
is set up via a PHPUnit bootstrap file without a custom entry point.

You can execute these tests by running:
$ vendor/bin/phpunit -d memory_limit=512M -c tests/phpunit/unit-tests.xml

Bug: T89432
Bug: T87781
Bug: T84948
Change-Id: Iad01033a0548afd4d2a6f2c1ef6fcc9debf72c0d