tests: add a test 055 for md

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
md: try hard to link stale objects during node events

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
tests: add 056 to test disk failure with node event change

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>