Mock out the network call (which you'd be doing anyway because unit tests never connect to the network) so it always returns a failure. Mock out the timer. Cal

Mock out the network call (which you'd be doing anyway because unit tests never connect to the network) so it always returns a failure. Mock out the timer. Call the function multiple times and ensure it'd passing the expected wait durations in for each time it's called followed by a fatal error after say 30 seconds.