test(gke): add unit tests for node pool reconcile diff checks

Table-driven tests for checkDiffAndPrepareUpdateConfig and
checkDiffAndPrepareUpdateSize covering the cases fixed in the preceding
commits: resource label sync via instance template (present, missing,
unavailable), Kubernetes label diff, image type case-insensitive match,
network tags, node locations, autoscaling suppression, nil Scaling guard,
and regional cluster replica division.

Author: jonathanrainer