add unit-test for Volume Core's VolumePath method

[gfl699468]

Ⅰ. Describe what this PR did

add unit-test for Volume Core's VolumePath method

Ⅱ. Does this pull request fix one issue?

fix #1762

Ⅲ. Describe how you did it

Ⅳ. Describe how to verify it

run
go test ./storage/volume/core_test.go ./storage/volume/core.go ./storage/volume/core_util.go ./storage/volume/config.go

Ⅴ. Special notes for reviews

baiji group 1

[pouchrobot]

@gfl699468 Thanks for your contribution. 🍻
Please sign off in each of your commits.

[CLAassistant]

All committers have signed the CLA.

[pouchrobot]

We found this is your first time to contribute to Pouch, @gfl699468
👏 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 #1788 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1788     +/-   ##
========================================
+ Coverage    56.3%   56.4%   +0.1%     
========================================
  Files         200     200             
  Lines       15657   15657             
========================================
+ Hits         8816    8832     +16     
+ Misses       5745    5733     -12     
+ Partials     1096    1092      -4

Flag	Coverage Δ
#critest	33.46% <ø> (-0.02%)	⬇️
#integrationtest	37.71% <ø> (+0.05%)	⬆️
#unittest	20.4% <ø> (+0.07%)	⬆️

Impacted Files	Coverage Δ
daemon/logger/jsonfile/utils.go	71.54% <0%> (-1.63%)	⬇️
daemon/mgr/container.go	53.3% <0%> (+0.15%)	⬆️
cri/v1alpha2/cri.go	66.13% <0%> (+0.17%)	⬆️
storage/volume/core.go	60.48% <0%> (+0.97%)	⬆️
ctrd/container.go	50.17% <0%> (+1.37%)	⬆️
ctrd/image.go	81.18% <0%> (+4.45%)	⬆️

[sunyuan3]

LGTM