For consistency with the PostgreSQL behavior this test program is
intended to simulate, use pwrite() instead of lseek() + write().

Also fix the final "non-sync" test, which was opening and closing the
file for every write.

Discussion: https://postgr.es/m/CA%2BhUKGJjjid2BJsvjMALBTduo1ogdx2SPYaTQL3wAy8y2hc4nw%40mail.gmail.com