Fix unittest race condition while checking callback requests

[Crsi]

b1c3843 introduced a quick fix to prevent failing API unittests due to race conditions with the callback check, because the evaluation of the received callbacks may be faster than receiving the callbacks from the server. Therefore, the API unittest would fail, even if the callback was actually happening and correctly performed.
That fix uses a static delay using time.sleep(), which is pretty ugly (but, at least it works).