tests: change type of sched_nice field to signed in sched_xetattr test
tests: change type of sched_nice field to signed in sched_xetattr test

Kernel headers declare this field as s32, and strace prints it with %d
specifier.

* tests/sched_xetattr.c (main): Change type of sched_nice field of struct
sched_attr to int32_t, update format specifiers accordingly.

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