tests: check decoding of sched_[gs]etattr corner cases

tests: check decoding of sched_[gs]etattr corner cases

* tests/sched_xetattr.c (main): Check that integer arguments
of sched_getattr and sched_setattr syscalls are decoded properly
by adding a few cases with filled higher 32 bits.
Check that pointer argument is decoded properly
on F8ILL_KULONG_SUPPORTED architectures.