Fair enough, but why would that matter enough for you to care? It's not going to matter for the test, and might cause problems, so why not use a more reasonable value? I would even argue against using private IP addresses at all on the WAN side for the same reason -- the firmware might be "smart" enough to object.
I agree. I just have found it best to have tests mimic the real world as much as possible, this example being a case in point.