Replace go mock with client-go fake client in unit tests

[tariq-hasan]

What you would like to be added?

go mock should be replaced with client-go fake client in unit tests.

Why is this needed?

Reference: #2289 (comment)

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Give it a 👍 We prioritize the features with most 👍

[github-actions]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[andreyvelich]

/remove-lifecycle stale
/good-first-issue
/remove-label lifecycle/needs-triage
/area testing

[google-oss-prow]

@andreyvelich:
This request has been marked as suitable for new contributors.

Please ensure the request meets the requirements listed here.

If this request no longer meets these requirements, the label can be removed
by commenting with the /remove-good-first-issue command.

In response to this:

/remove-lifecycle stale
/good-first-issue
/remove-label lifecycle/needs-triage
/area testing

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