tests/sched_xetattr.c: use libtests
tests/sched_xetattr.c: use libtests

* tests/sched_xetattr.c: Use SKIP_MAIN_UNDEFINED.
(main) Use perror_msg_and_skip.

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