test: Avoid sem_timedwait() in test_max_buf
test: Avoid sem_timedwait() in test_max_buf

- Use pthread_cond_timedwait() instead of sem_timedwait(), since
sem_timedwait() is not available on all platforms (e.g. NetBSD)

cmusatyalab/minirpc,cmusatyalab/minirpc