Improve unit test coverage in webhooks

[willie-yao]

/kind cleanup

What needs cleanup:

This mega issue is meant to track individual issues for improving code coverage within CAPZ webhooks.

Describe the solution you'd like

### Tasks
- [ ] #3371
- [ ] #3652
- [ ] #3372
- [ ] #3374
- [ ] #3373
- [ ] #3429
- [ ] #3430
- [ ] #3431
- [ ] azuremanagedmachinepool_webhook.go
- [ ] #3432
- [ ] azuremachinepool_webhook.go
- [ ] azuremachinepoolmachine_webhook.go

Older parent issue: #3229

[k8s-triage-robot]

The Kubernetes project currently lacks enough contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

• Mark this issue as fresh with /remove-lifecycle stale
• Close this issue with /close
• Offer to help out with Issue Triage

Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle stale

[k8s-triage-robot]

The Kubernetes project currently lacks enough active contributors to adequately respond to all issues.

This bot triages un-triaged issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 30d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 30d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

• Mark this issue as fresh with /remove-lifecycle rotten
• Close this issue with /close
• Offer to help out with Issue Triage

Please send feedback to sig-contributor-experience at kubernetes/community.

/lifecycle rotten

[willie-yao]

/remove-lifecycle rotten