Running module unit tests from CLI: "Class '..' could not be found"
Ok, so the solution is

From Drupal root directory:

sudo -u www-data php core/scripts/run-tests.sh --url http://d8.localhost/ --class "Drupal\system\Tests\Installer\InstallerTranslationTest"


Assuming that your site is available under http://d8.localhost, and it is an installed site.

Maybe you want to use --verbose for more output.

Documentation on drupal.org: Running Tests Through command-line