tests: add more tests for sched_getscheduler and sched_xetscheduler
tests: add more tests for sched_getscheduler and sched_xetscheduler

* tests/sched_xetscheduler.c (main): Check for decoding of invalid PID
in sched_getscheduler and sched_setscheduler, invalid address
of sched_param structure, and invalid policy value.

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