Interesting approach, not what'd I'd have done. I would simply have created a tests/autoloader.php that has something like

    $autoLoader = require_once( __DIR__ . '/../vendor/autoload.php' );
    $autoLoader->addPsr4( 'Wikibase\\Test\\', __DIR__ . '/unit/' );

That can then be included by both tests/bootstrap.php and the test bootstrap of any extension to SMW.