Fix the G5k tests to avoid false negatives
regression.sh can fail when the g5k tests are passing, simply because the main code gets notified of group creation in the wrong order. This commit change the suite to NAK on exit code errors instead of outputs to try to fix this problem. This is a temporary fix at best, we should instead improve the tests so that the validation is more thorough, and results in crashes when something wrong happens. Related to issue #2.