[unit test] improve unit test coverage for yurthub

[rambohe-ch]

What would you like to be added:
improve unit test coverage for yurthub:

• current coverage: 45%
  https://app.codecov.io/gh/openyurtio/openyurt/compare/911/tree/pkg/yurthub

• target coverage: 70%

Why is this needed:
improve unit test coverage for OpenYurt project.

others
/kind feature

[Congrool]

Since we're pending at #882 which has great changes on yurthub cache components, could we write unit tests after #882 being merged? Otherwise we have to rewrite these tests later.

Also, we'd better do #857 based on #882, otherwise we have to retest it later.

[rambohe-ch]

Since we're pending at #882 which has great changes on yurthub cache components, could we write unit tests after #882 being merged? Otherwise we have to rewrite these tests later.

Also, we'd better do #857 based on #882, otherwise we have to retest it later.

@Congrool ok, i will focus on reviewing #882 and merge this pull request before unit test.

[donychen1134]

/assign donychen1134