Because it allows you to run unit tests when the library is installed as part of composer.

Oftentimes I debug libraries while they are installed in composer by composing them but you want access to the whole vendor library you're running in and that's what this bootstrap does.  I actually simplified the zfcampus bootstrap by a lot to put this here.  https://github.com/zfcampus/zf-apigility/blob/master/test/Bootstrap.php

You'l also see the latest trend in PHP, as zfcampus is controlled by MWOP, that the unit test dir is test and not tests