refactor: replace gomock with fake client in generator_test.go

[haroon0x]

What this PR does / why we need it:

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #2408

Checklist:

• Docs included if any changes are user facing

[github-actions]

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