Despite our efforts, sometimes the async cancellation gets deferred.
Notice this by calling pthread_testcancel(), and then try to work out if
async cancellation was ever successful by checking if all threads ran
for the full expected time, or if some were stopped early.
Also, increase the time we allow for the async cancellation to get
delivered to 30 seconds.
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>