fixes #1761 baiji-group=264-3

[jiapengcs]

Ⅰ. Describe what this PR did

Add unit-test for Volume Core's DetachVolume method which locate on storage/volume/core.go.

Ⅱ. Does this pull request fix one issue?

fixes #1761

Ⅲ. Describe how you did it

implement function TestDetachVolume in core_test.go

Ⅳ. Describe how to verify it

unit test, run go test -test.run DetachVolume

Ⅴ. Special notes for reviews

None

[pouchrobot]

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

[pouchrobot]

We found this is your first time to contribute to Pouch, @jiapengcs
👏 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 @jiapengcs
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 !

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@f369ee7). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1890   +/-   ##
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  Coverage          ?   56.49%           
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  Files             ?      200           
  Lines             ?    15659           
  Branches          ?        0           
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  Hits              ?     8846           
  Misses            ?     5722           
  Partials          ?     1091

Flag	Coverage Δ
#critest	33.54% <ø> (?)
#integrationtest	37.65% <ø> (?)
#unittest	20.59% <ø> (?)