On Moodle 3.0+ (Build: 20151209), version 2015111600.04 if you initialise PHPUnit (on Windows) with:
then you get the following error:
looking at the line it has:
This is not defined in the $CFG global or is it mentioned in config-dist.php. If it would is auto generated when installing then for those who upgrade and copy their config.php file there is no indication about adding it.