Make valid_model measurement more stable, otherwise StarPU would flush them and non-linear regression complain

tests/perfmodels/valid_model.c

@@ -27,6 +27,7 @@ void func(void *descr[], void *arg)
 {
 	(void)descr;
 	(void)arg;
+	usleep(1000);
 }
 
 static struct starpu_perfmodel rb_model =