improve unit test coverage for yurtappdaemon

[rudolf-chy]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line: /kind bug /kind documentation /kind enhancement /kind good-first-issue /kind feature /kind question /kind design /sig ai /sig iot /sig network /sig storage /sig storage

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

other Note

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In response to [this](https://github.com/openyurtio/yurt-app-manager/pull/106): > > > >#### What type of PR is this? >> Uncomment only one ` /kind <>` line, hit enter to put that in a new line, and remove leading whitespace from that line: >> /kind bug >> /kind documentation >> /kind enhancement >> /kind good-first-issue >> /kind feature >> /kind question >> /kind design >> /sig ai >> /sig iot >> /sig network >> /sig storage >> /sig storage > > > >#### What this PR does / why we need it: > >#### Which issue(s) this PR fixes: > >Fixes # > >#### Special notes for your reviewer: > > > >#### Does this PR introduce a user-facing change? > >```release-note > >``` > >#### other Note > > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[codecov[bot]]

Codecov Report

Merging #106 (2cc2a86) into master (69c860f) will increase coverage by 0.08%. The diff coverage is n/a.

:exclamation: Current head 2cc2a86 differs from pull request most recent head c0567c4. Consider uploading reports for the commit c0567c4 to get more accurate results

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   23.45%   23.53%   +0.08%     
==========================================
  Files          42       42              
  Lines        4776     4776              
==========================================
+ Hits         1120     1124       +4     
+ Misses       3504     3500       -4     
  Partials      152      152              

Flag	Coverage Δ
unittests	23.53% <ø> (+0.08%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...troller/yurtappdaemon/nodepool_enqueue_handlers.go	20.00% <0.00%> (+20.00%)	:arrow_up:

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[rambohe-ch]

@Rudolf1998 please fix the golangci-lint errors.

[rambohe-ch]

/lgtm /approve

[openyurt-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rambohe-ch, Rudolf1998

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/openyurtio/yurt-app-manager/blob/master/OWNERS)~~ [rambohe-ch] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment