kubelet/volumeManager: improve unit test coverage

[TommyStarK]

/kind cleanup

What this PR does / why we need it:

If applied this commit will improve unit test coverage of pkg/kubelet/volumemanager.

• before

• after

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: TommyStarK
Once this PR has been reviewed and has the lgtm label, please assign msau42 for approval. For more information see the Kubernetes Code Review Process.

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[TommyStarK]

I'm trying to get involved and contribute again to K8s, so to get back on track I thought I'd start by improving unit test code coverage. I apologize in advance if some of the tests/cases added are irrelevant. I lack a lot of knowledge in this area. I hope it will be useful anyway.

[bart0sh]

/triage accepted
/priority important-longterm