🌱 Add UTs for failure domain in KCP

[arshadd-b]

(🌱 )

What this PR does / why we need it:
This improves the coverage for failure domain in KCP

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 # #11602

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Hi @arshadd-b. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[arshadd-b]

Hi @fabriziopandini
I have noticed that in control_plane.go file there are no test cases for these functions
https://github.com/kubernetes-sigs/cluster-api/blob/main/controlplane/kubeadm/internal/control_plane.go#L325
https://github.com/kubernetes-sigs/cluster-api/blob/main/controlplane/kubeadm/internal/control_plane.go#L355
This PR doesn't cover these , Is it expected to cover these as well ?
I can raise a different PR covering these functions
Thanks

[chrischdi]

/ok-to-test