Bootstrap: Use root_path with __DIR__ to get the correct path

When using PHPUnit tests with some IDE, there can be error with the tests suites.

My IDE for example has its own PHPUnit integrated. When I execute the phpunit.xml.dist then, it loads the boostrap, finds line
$phpbb_root_path = '/phpBB/';
and load the require_once() files after that from the wrong folder, cause they are relative to the file wich is executed first.

My suggested solution is using
$phpbb_root_path = _DIR_ . '/../phpBB/';
_DIR_ is shortand for the directory where the current file (so bootstrap.php) is placed.