If you surf inside the Frameworks/test/CTestCase.php, then you will find like this,
require_once('PHPUnit/Runner/Version.php');
require_once('PHPUnit/Util/Filesystem.php'); // workaround for PHPUnit <= 3.6.11
spl_autoload_unregister(array('YiiBase','autoload'));
require_once('PHPUnit/Autoload.php');
spl_autoload_register(array('YiiBase','autoload')); // put yii's autoloader at the end
if (in_array('phpunit_autoload', spl_autoload_functions())) { // PHPUnit >= 3.7 'phpunit_autoload' was obsoleted
spl_autoload_unregister('phpunit_autoload');
Yii::registerAutoloader('phpunit_autoload');
}
I think, it will help you to understand your problem.