at_exit: don't mask nonzero exit status due to $!
Created by: sunaku
This change ensures that RSpec's at_exit handler doesn't mask (override) any nonzero exit status, which can happen when Ruby is on it's way out due to an uncaught exception ($!) such as a SyntaxError.
Please see this bug report for more information:
https://github.com/sunaku/tork/issues/31#issuecomment-3801441