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

tests/perfmodels/user_base.c

@@ -26,6 +26,7 @@ void func(void *descr[], void *arg)
 {
 	(void)descr;
 	(void)arg;
+	usleep(1000);
 }
 
 size_t get_size_base(struct starpu_task *task, unsigned nimpl)