tests/userfaultfd.c: use libtests
tests/userfaultfd.c: use libtests

* tests/userfaultfd.c: Use SKIP_MAIN_UNDEFINED.
(main): Use assert.

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