[eluser]bEz[/eluser]
If your datamapper model is Test and assuming you're following convention, there is no 'testid' field, just an 'id' field.
You therefore would not need to "VALIDATE" the 'id' field. You would, however, add the 'id' field to the form fields array.
Now, if there was some other use of the field testid, then you could define the type as hidden via the $form_fields array (can't recall if this still requires a custom function in the Model declaration).