We have phpunit.xml.dist in the root of the MediaWiki repo, and it's intended to use this file with vendor/bin/phpunit (or composer phpunit) rather than the PHPUnit MediaWiki maintenance class at tests/phpunit/phpunit.php.
In order to phase out the usage of tests/phpunit/suite.xml, we need to ensure that all tests currently run via the maintenance class can be run with the plain phpunit entrypoint. We also need to ensure that the test files are located in directories expected by the test suites defined in phpunit.xml.dist (e.g. unit and integration directories).