Have you looked at web test tools like Selenium? Since the custom and XaaS forms are just webpages you should be able to have Selenium walk the form and validate drop-down relationships etc.
I'd also add that for my XaaS forms where I use a vRO action to handle constraints etc I'd observe that these relationships can be viewed as "chains" where the changing a value should cause the chain re-compute. So, I use a workflow to drive values at the chains - sort of of like unit testing where I have to handle chain order execution, validation of expected results etc.
Anyway, HTH