Для повторения итераторов вы можете использовать класс AppendIterator.
$directoryIterator = new RecursiveDirectoryIterator( __DIR__ . '/tests/phpunit/' );
$directoryIterator1 = new RecursiveDirectoryIterator( __DIR__ . '/tests/integration/' );
$iterator = new AppendIterator();
$iterator->append(new RecursiveIteratorIterator( $directoryIterator ));
$iterator->append(new RecursiveIteratorIterator( $directoryIterator1 ));
/**
* @var SplFileInfo $fileInfo
*/
foreach ($iterator as $fileInfo) {
if ( substr( $fileInfo->getFilename(), -8 ) === 'Test.php' ) {
$files[] = $fileInfo->getPathname();
}
}