Azure: Fix a flaky test for azure vmss.

[nilo19]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind api-change
/kind bug
/kind cleanup
/kind deprecation
/kind design
/kind documentation
/kind failing-test
/kind feature

/kind flake

What this PR does / why we need it:
The test TestEnsureBackendPoolDeletedConcurrently is flaky because there's a potential race condition if multiple goroutines are working on the same VMSS. This PR fixes it and test the performance of EnsureBackendPoolDeleted on multiple scale sets concurrently.

Which issue(s) this PR fixes:

Fixes #90816

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

/area provider/azure
/priority important-soon
/sig cloud-provider

[andyzhangx]

/test pull-kubernetes-e2e-gce-100-performance
/test pull-kubernetes-e2e-azure-disk-windows

[feiskyer]

/lgtm
/approve
/retest

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: feiskyer, nilo19

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• staging/src/k8s.io/legacy-cloud-providers/azure/OWNERS [feiskyer]

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