Fix timespec_from_nsec test for 32-bit time_t

Since struct timespec's tv_sec member is of type time_t, adjust the
expected value to allow for the truncation which will occur with 32-bit
time_t.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
(cherry picked from commit dd1dba80)
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2043