Functional Tests with TYPO3 v10 no composer autoload

When I try to run a functional test which uses the class TYPO3\CMS\Frontend\Tests\Functional\SiteHandling\AbstractTestCase I get an error Class 'TYPO3\CMS\Frontend\Tests\Functional\SiteHandling\AbstractTestCase' not found.... Is this an composer autoloader problem? Any Ideas how to fix this?

/usr/bin/php7.4 /mypath/typo3_10/vendor/phpunit/phpunit/phpunit --bootstrap /mypath/typo3_10/vendor/typo3/testing-framework/Resources/Core/Build/FunctionalTestsBootstrap.php --configuration /mypath/typo3_10/vendor/typo3/testing-framework/Resources/Core/Build/FunctionalTests.xml /mypath/typo3_10/web/typo3/sysext/seo --teamcity

is the command in PHPStorm.