As you might now, I am working as a Software Development Engineer in Test at Microsoft Development Center in Denmark.
But the only real test framework that comes with AX is the SysTest framework (you can read more about it here).
Below you will find another framework for Dynamics AX, that was written by one of the members on my test team and allows to implement and use mock objects for testing the AX application.
More information and download link
It would be great if you could try it out and provide your feedback here or directly on the author's blog.