Increase validation test coverage

[johngmyers]

More prep work for rolling update improvements.

[k8s-ci-robot]

Hi @johngmyers. Thanks for your PR.

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[rifelpet]

/ok-to-test

[rifelpet]

Thanks for the improvement! I think it would be nice if we created enum-style constants for ValidationError.Kind which would allow us to more easily perform meaningful assertions beyond just the length of v.Failures but that can be done later.

before:

ok      k8s.io/kops/pkg/validation      0.821s  coverage: 46.3% of statements

after:

ok      k8s.io/kops/pkg/validation      0.801s  coverage: 68.6% of statements

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johngmyers, rifelpet

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