tests/sendfile.c: use libtests
tests/sendfile.c: use libtests

* tests/sendfile.c: Use SKIP_MAIN_UNDEFINED.
(main): Use assert, perror_msg_and_fail, perror_msg_and_skip,
and tail_alloc.

cuviper/strace,cuviper/strace,cuviper/strace,Saruta/strace,Saruta/strace,cuviper/strace,Saruta/strace,Saruta/strace,Saruta/strace,cuviper/strace