As far as I know, this is not possible with only NodeUnit, since it uses an asynchronous approach where the callbacks don't execute in a predictable order. This behavior is by desing, and is supposed to provide a more realistic testing routines and prevent dependencies between tests.
You can however use Grunt JS with the NodeUnit plugin to take full control of your testing process.
You can find a great article on this topic here:
http://markdalgleish.com/2012/09/test-driven-node-js-development-with-grunt/