You need a fully working nextcloud/owncloud instance. Check-out this app into the apps folder, and follow the instructions below. The tests will modify your database but should automatically revert the changes in the cleanup stage. (As provided by https://github.com/ChristophWurst/nextcloud_testing)
cd <nextcloud root>
sudo -u php occ app:enable registration
If you don't have composer installed yet, follow https://getcomposer.org/download/
Then install dev dependencies:
composer install
sudo -u www-data vendor/bin/phpunit -c tests/phpunit.unit.xml
sudo -u www-data vendor/bin/phpunit -c tests/phpunit.integration.xml