test: threads: Use pthread_testcancel()

`pthread_testcancel()` is a guaranteed cancellation point,
specifically for testing if cancellation has been requested,
so use it.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

test/threads.cpp

@@ -52,14 +52,8 @@ protected:
 	{
 		cancelled_ = true;
 
-		/*
-		 * Cancel the thread and call a guaranteed cancellation point
-		 * (nanosleep).
-		 */
 		pthread_cancel(pthread_self());
-
-		struct timespec req{ 0, 100*000*000 };
-		nanosleep(&req, nullptr);
+		pthread_testcancel();
 
 		cancelled_ = false;
 	}