For every patch handler, implement a tests verifies the handler is idempotent

[dlipovetsky]

Idempotence: An External Patch Extension must only return patches if changes to the templates are required, i.e. unnecessary patches when the template is already in the desired state must be avoided.
-- https://cluster-api.sigs.k8s.io/tasks/experimental-features/runtime-sdk/implement-topology-mutation-hook#patch-extension-guidelines

This is especially important any patch that appends to an array, since a patch that is not idempotent will append redundant elements to the array when it is called two or more times.