PHP Tools come with a bundled version of PHPUnit in case there isn't a user installation of PHPUnit in the project.
PHP Tools will use a particular PHPUnit version when it's configured in the
composer.json. Check Composer section, and ensure there is
phpunit/phpunit composer package installed.
In case there is no
phpunit/phpunit composer package, the Test Explorer will use bundled PHPUnit; a stable version of PHPUnit chosen accordingly for the PHP version configured in the Project Properties.