To make sure you can step debug run: export PHP_IDE_CONFIG="serverName=dev.portal.local"
in your terminal before running tests.
Also make sure you have this server configured in the PHPStorm Settings -> PHP -> Servers.
To disable warnings when IDE is not listening for connections do this, for example:
export XDEBUG_MODE=off && vendor/bin/phpunit tests/Feature/Test.php
To turn it on again do this, for example:
export XDEBUG_MODE=develop,debug && vendor/bin/phpunit tests/Feature/Test.php