Previously, the selftest framework always treats it as *ok* even though
some of them are failed actually. That's because the script always
returns 0.

It supports PASS/FAIL/SKIP exit code now.

CC: Philip Li <philip.li@intel.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <zhijianx.li@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>