test: add unit test for function ValidateOOMScore in oom_score_test.go

[quyi1993]

Ⅰ. Describe what this PR did

Ⅱ. Does this pull request fix one issue?

fixes #1639

Ⅲ. Describe how you did it

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[CLAassistant]

All committers have signed the CLA.

[pouchrobot]

We found this is your first time to contribute to Pouch, @quyi1993
👏 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]

@quyi1993 Thanks for your contribution. 🍻
Please sign off in each of your commits.

[codecov-io]

Codecov Report

Merging #1658 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1658      +/-   ##
==========================================
- Coverage   41.16%   41.03%   -0.13%     
==========================================
  Files         274      274              
  Lines       18091    18091              
==========================================
- Hits         7447     7424      -23     
- Misses       9729     9758      +29     
+ Partials      915      909       -6

Impacted Files	Coverage Δ
pkg/meta/store.go	54.76% <0%> (-4.77%)	⬇️
daemon/containerio/cio.go	58.73% <0%> (-4.77%)	⬇️
daemon/containerio/container_io.go	53.59% <0%> (-4.42%)	⬇️
daemon/mgr/system.go	73.27% <0%> (-3.45%)	⬇️
ctrd/image.go	75.27% <0%> (-2.75%)	⬇️
daemon/mgr/container_utils.go	54.97% <0%> (-1.74%)	⬇️
daemon/mgr/container.go	50.23% <0%> (+0.15%)	⬆️
daemon/logger/jsonfile/utils.go	73.17% <0%> (+1.62%)	⬆️
apis/server/utils.go	66.66% <0%> (+4.76%)	⬆️
apis/opts/oom_score.go	100% <0%> (+100%)	⬆️

[faremax]

issue #1639

[allencloud]

LGTM