[libc] Fix test passing negative value in timespec passed to nanosleep (#65346)

This test was setting tv_nsec to a negative value, which as per the
standard this is an EINVAL:

The value in the tv_nsec field was not in the range [0, 999999999] or
tv_sec was negative.

https://man7.org/linux/man-pages/man2/nanosleep.2.html