Add test coverage for adding/removing FD in KCP

[fabriziopandini]

While implementing #11598 I noticed that test coverage for failure domain management in KCP is minimal or not existent.

The PR partially addressed the tech debt by implementing test coverage for the failuredomains package (which was modified by the PR), but the upper layer of the call stack (scale.go, control_plane.go) still lack of test coverage, mostly with regards of scenarios where we are adding or removing failure domains.

/kind cleanup
/area provider/control-plane-kubeadm

[arshadd-b]

/assign

[arshadd-b]

I am interested, will work on this

[chrischdi]

/assign @arshadd-b

/triage accepted
/priority important-soon