These tests async thread cancellation of a thread that doesn't have any
cancellation points.
Unfortunately, since 2b165a45 the async cancellation silently fails when
the thread is inside the kernel function Sleep(), so it just exits
normally after 10 seconds. (See the commentary in pthread::cancel() in
thread.cc, where it checks if the target thread is inside the kernel,
and silently converts the cancellation into a deferred one)
Work around this by busy-waiting rather than Sleep()ing for 10 seconds.
This is still somewhat fragile: the async cancel could still fail, if it
happens to occur while we're inside the kernel function that time()
calls.
v2:
Do nothing more efficiently
Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
