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.