test: TestRemoveVolume

[mengjiahao]

Ⅰ. Describe what this PR did

#1633 add unit-test for RemoveVolume

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. Describe how you did it

NONE

Ⅳ. Describe how to verify it

NONE

Ⅴ. Special notes for reviews

NONE

[pouchrobot]

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

[pouchrobot]

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

[codecov-io]

Codecov Report

Merging #1679 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1679      +/-   ##
==========================================
- Coverage   41.16%   41.15%   -0.01%     
==========================================
  Files         274      274              
  Lines       18091    18091              
==========================================
- Hits         7447     7446       -1     
- Misses       9729     9732       +3     
+ Partials      915      913       -2

Impacted Files	Coverage Δ
ctrd/image.go	75.27% <0%> (-2.75%)	⬇️
daemon/mgr/system.go	75% <0%> (-1.73%)	⬇️
storage/volume/core.go	58.53% <0%> (+1.95%)	⬆️
apis/server/utils.go	66.66% <0%> (+4.76%)	⬆️

[allencloud]

LGTM