tests: skip stat32 test if struct stat is defined incorrectly

tests: skip stat32 test if struct stat is defined incorrectly

If stat.st_mode returned by syscall is 0, it means that the definition
of struct stat in <asm/stat.h> is incorrect.  This is the case e.g. on
mips n32 where the only available stat syscall is 64-bit but at the same
time struct stat defined by <asm/stat.h> is 32-bit.

* tests/stat.c (main) [NR_stat]: Return 77 if st_mode is 0.