tests/xstatx.c: fix stat syscall tests on mips64

tests/xstatx.c: fix stat syscall tests on mips64

For historical reasons the kernel struct stat represents times as unsigned
32-bit integers on mips64. Therefore, while it's possible to give a file a
timestamp before 1970 with futimens, reading the same timestamp through
struct stat will give a positive time (around 2106).
Workaround by using positive timestamps for testing on mips64.

* tests/xstatx.c (create_sample): Use positive timestamps on mips64.