The test API is not hooked up by default to a test name server that you can perform digs against.
In other words, the name server is not listening for changes to your test zones.
We could configure it to have one more of your zones and, at that point, you could test for changes.
But we would need to configure it first. Our engineering team does that so it might take 2-3 days.
Once that's done, we could provide the name server where your test zone replicates to so that you could test it.
Contact Customer Support and let us know the names of any zones you want to add.