test: add unit-tests for core AttachVolume

[forienlauo]

Ⅰ. Describe what this PR did

#1635: add unit-tests for core AttachVolume

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]

We found this is your first time to contribute to Pouch, @forienlauo
👏 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 #1688 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #1688      +/-   ##
=========================================
+ Coverage   41.16%   41.2%   +0.03%     
=========================================
  Files         274     274              
  Lines       18091   18091              
=========================================
+ Hits         7447    7454       +7     
+ Misses       9729    9727       -2     
+ Partials      915     910       -5

Impacted Files	Coverage Δ
daemon/mgr/system.go	75% <0%> (-1.73%)	⬇️
ctrd/image.go	77.47% <0%> (-0.55%)	⬇️
daemon/mgr/container.go	50.23% <0%> (+0.15%)	⬆️
daemon/logger/jsonfile/utils.go	73.17% <0%> (+1.62%)	⬆️
storage/volume/core.go	58.53% <0%> (+1.95%)	⬆️
apis/server/utils.go	66.66% <0%> (+4.76%)	⬆️

[allencloud]

PTAL @rudyfly @shaloulcy
Thanks a lot.

[rudyfly]

LGTM