Issues settings up drush + phpunit
Alright so here's what I did, with help from MegaChriz.

The dev dependencies for Drush need to be installed and the phpunit version that comes with Drush should be used instead of a 'general' phpunit. So not the one I installed through composer.


Git clone of Drush from my home folder (/users/wim.mostrey)

git clone https://github.com/drush-ops/drush.git
cd drush
git checkout 8.1.9

Install Drush with dev dependencies using Composer while being inside the Drush folder

composer install

Execute built-in tests pointing to the phpunit version from Drush:

UNISH_NO_TIMEOUTS=1 /users/wim.mostrey/drush/vendor/bin/phpunit --configuration /users/wim.mostrey/drush/tests --filter=ImageCase


or for Drupal 7 tests:

UNISH_NO_TIMEOUTS=1 UNISH_DRUPAL_MAJOR_VERSION=7 /users/wim.mostrey/drush/vendor/bin/phpunit --configuration /users/wim.mostrey/drush/tests /users/wim.mostrey/Sites/drupal7/sites/all/modules/feeds/tests/drush