See T266389 for context.
For example, with AbuseFilter, there are tests in tests/phpunit/unit which access MediaWiki services and globals which are otherwise prohibited in MediaWikiUnitTestCase. If you try to run these locally with vendor/bin/phpunit they fail:
composer phpunit:unit -- --exclude-group Broken,ParserFuzz,Stub --stop-on-failure There was 1 error: 1) AbuseFilterParserTest::testDUNDEFINED with data set #6 ('norm(user_name) !== rmspecials('')') Error: Class 'Wikimedia\Equivset\Equivset' not found /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:1610 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:1677 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:1219 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:1016 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:938 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:880 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:869 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:825 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:815 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:780 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:756 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:727 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:695 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:661 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:607 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:533 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:442 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:423 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:404 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:226 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php:244 /home/kostajh/src/mediawiki/core/extensions/AbuseFilter/tests/phpunit/unit/AbuseFilterParserTest.php:978 /home/kostajh/src/mediawiki/core/tests/phpunit/MediaWikiUnitTestCase.php:112
But, in CI no error is thrown with the same command: https://integration.wikimedia.org/ci/job/quibble-vendor-mysql-php72-noselenium-docker/42693/consoleFull#console-section-9
I don't understand how or why that is happening, but it's supposed to throw an error. IIRC there was a task around this although the only thing I find at the moment is T229220: Unit tests are not being run for extensions under PHPUnit 4.x (HHVM) which is tangential. Pinging @hashar for ideas or clues.