This is a parent task to collect instances where tests fail when run on their own, either by passing the class as an argument to the phpunit entrypoint or by using --filter to narrow down to a specific test method in the class.
Background: there are currently several tests in MediaWiki core and extensions/skins that pass in CI, because they are executed in the same process as other tests (using the entrypoint php tests/phpunit/phpunit.php or php tests/phpunit/phpunit.php --testsuite=extensions). Those tests should be able to pass when run separately from the others.