tests: change command 'hd' to 'hexdump'

In rhel6/rhel5, we don't have command 'hd' but only have 'hexdump',
so using 'hexdump -C' instead of 'hd' is a better way.

Signed-off-by: Robin Dong <sanbai@taobao.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>