[IMPROVEMENT] Implement/fix the unit tests of Volume Attachment and volume controller

[PhanLe1010]

Implement/fix unit tests of Volume Attachment and Volume controller

https://github.com/longhorn/longhorn-manager/blob/master/controller/volume_attachment_controller_test.go
https://github.com/longhorn/longhorn-manager/blob/master/controller/volume_controller_test.go

[longhorn-io-github-bot]

Pre Ready-For-Testing Checklist

• Where is the reproduce steps/test steps documented?
  The reproduce steps/test steps are at: [IMPROVEMENT] Implement/fix the unit tests of Volume Attachment and volume controller #6005 (comment)

• Is there a workaround for the issue? If so, where is it documented?
  The workaround is at:

• Does the PR include the explanation for the fix or the feature?

• Does the PR include deployment change (YAML/Chart)? If so, where are the PRs for both YAML file and Chart?
  The PR for the YAML change is at:
  The PR for the chart change is at:

• Have the backend code been merged (Manager, Engine, Instance Manager, BackupStore etc) (including backport-needed/*)?
  The PR is at Implement/fix the unit tests of Volume Attachment and volume controller longhorn-manager#2053

• Which areas/issues this PR might have potential impacts on?
  Area Image Build
  Issues

• If labeled: require/LEP Has the Longhorn Enhancement Proposal PR submitted?
  The LEP PR is at

• If labeled: area/ui Has the UI issue filed or ready to be merged (including backport-needed/*)?
  The UI issue/PR is at

• If labeled: require/doc Has the necessary document PR submitted or merged (including backport-needed/*)?
  The documentation issue/PR is at

• If labeled: require/automation-e2e Has the end-to-end test plan been merged? Have QAs agreed on the automation test case? If only test case skeleton w/o implementation, have you created an implementation issue (including backport-needed/*)
  The automation skeleton PR is at
  The automation test case PR is at
  The issue of automation test case implementation is at (please create by the template)

• If labeled: require/automation-engine Has the engine integration test been merged (including backport-needed/*)?
  The engine automation PR is at

• If labeled: require/manual-test-plan Has the manual test plan been documented?
  The updated manual test plan is at

• If the fix introduces the code for backward compatibility Has a separate issue been filed with the label release/obsolete-compatibility?
  The compatibility issue is filed at

[innobead]

Also, need to backport this to 1.5.x even if its unit tests because it will help with the PR check.

[PhanLe1010]

Test plan:
Just make sure that longhorn-manager image is built successfully

[roger-ryao]

Referring to #6303 (comment), it was confirmed through https://drone-pr.longhorn.io/longhorn/longhorn-manager/5596/1/3 that the issue has been resolved.