I have used manually written as well as Grails generated Unit tests for this command object:
package myapp @grails.validation.Validateable class
I believe I found the grails supported way to unit test Command objects in grails 2.0. You need to use mockCommandObject provided by the ControllerUnitTestMixin.
Credit to Erik
http://www.jworks.nl/2012/04/12/testing-command-objects-in-grails-2-0/