Baiji issue 6

[lzpawell]

Ⅰ. Describe what this PR did

TestDetachVolume

Ⅱ. Does this pull request fix one issue?

fix #1761

Ⅲ. Describe how you did it

I add some code for testing method Core.DetachVolume
I make two test cases in this issue
case 1 is for testing condition that detach volume not exists or not attached
case 2 is for testing condition that detach an attached volume

Ⅳ. Describe how to verify it

the run Test cases an it can pass

Ⅴ. Special notes for reviews

In my work, I found that the parameter string[] labels is never used in the code, I suspect that is a bug

[pouchrobot]

Thanks for your contribution. 🍻 @lzpawell
While we thought PR TITLE could be more specific, longer than 20 chars.
Please edit this PR title instead of opening a new one.
More details, please refer to https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[pouchrobot]

@lzpawell Thanks for your contribution. 🍻
Please sign off in each of your commits.

[pouchrobot]

We found this is your first time to contribute to Pouch, @lzpawell
👏 We really appreciate it.
Just remind that you have read the contribution guide: https://github.com/alibaba/pouch/blob/master/CONTRIBUTING.md
If you didn't, you should do that first. If done, welcome again and please enjoy hacking! 🍻

[pouchrobot]

ping @lzpawell
Conflict happens after merging a previous commit.
Please rebase the branch against master and push it back again. Thanks a lot.

[YaoZengzeng]

#1824 which has been merged solves the same problem with this PR, so close it.

Thanks for your work !