Fix poll able streams for Darwin (and probably BSD)

Fix poll able streams for Darwin (and probably BSD)

Darwin's poll doesn't change revents if there are no available events, though it returns 0. Initialize the fd.revents to 0 so that the test passes.

That reveals a test failure, though, because with socket streams it takes time for an event to pass through the socket. Provide an 80-usec delay to allow time for the propagation.