[Umbrella] Improve test coverages

[kerthcet]

This is the currently unit test coverage:

∅  api/core/v1alpha1 (64ms)
∅  pkg (5ms)
∅  api/inference/v1alpha1 (43ms)
∅  pkg/controller (33ms)
∅  pkg/cert (78ms)
∅  pkg/controller/inference (28ms)
∅  pkg/webhook (23ms)
✓  pkg/controller_helper/model_source (1.014s) (coverage: 4.8% of statements)
✓  pkg/controller_helper (2.011s) (coverage: 3.6% of statements)
✓  pkg/util (3.184s) (coverage: 5.8% of statements)

We need to improve this.

[kerthcet]

/kind cleanup

[kerthcet]

/good-first-issue

[googs1025]

I am trying to get involved and familiar with this project, I am willing to help
/assign

[kerthcet]

Thanks @googs1025

[X1aoZEOuO]

I want to try it. @kerthcet

[kerthcet]

Feel free to take it.
/assign @X1aoZEOuO