Automatic merge from submit-queue (batch tested with PRs 62885, 62832). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

GCE: Fix ILB issue updating backend services

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #62878
`ensureInternalBackendServiceGroups` would sync the instance groups with the latest nodes, and update the backend service's list of backends if necessary. However, the list of backends on the backend service was not set before calling the GCP API.

`updateInternalLoadBalancerNodes` does very little above `ensureInternalBackendServiceGroups`, so I'm just combining the unit tests into one.

**Special notes for your reviewer**:
/assign MrHohn
cc @agau4779 

**Release note**:
```release-note
GCE: Fix for internal load balancer management resulting in backend services with outdated instance group links.
```