10535 sdk python test callback race

[jrandall]

fixes 10535

[jrandall]

Change 10.0s to a constant

[tomclegg]

I changed the notification mechanism here from threading.Condition() to threading.Event() in a983bbe, which (according to my reasoning, and a few trials) fixes the race and eliminates the need for sleep(). Still added the 10-second timeouts proposed here though, so if the code breaks then the tests fail instead of hanging. @jrandall if a983bbe (in master) works for you too, I think we can close this PR. Either way, thanks for pinning this down!

[tfmorris]

@jrandall I'm assuming that this fixed things, so closing the PR, but if not, please feel free to reopen.