tests/func/030: refine test to reflect default format parameter

Now we can restore 100% for 'cluster snapshot' so add test for it.

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
CHANGELOG.md: add 'cluster snapshot' enhancement description

Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>