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.

gio/gunixoutputstream.c

@@ -532,6 +532,7 @@ g_unix_output_stream_pollable_is_writable (GPollableOutputStream *stream)
 
   poll_fd.fd = unix_stream->priv->fd;
   poll_fd.events = G_IO_OUT;
+  poll_fd.revents = 0;
 
   do
     result = g_poll (&poll_fd, 1, 0);