make sure we test LoadBalancers at scale

#36720 removes the LoadBalancer tests from `gce-master-scale-correctness` because that job is actually too large for the tests to reliably work. We should still be regularly testing LoadBalancers at 1000 or 2000-node scale though. (Also, when we say "testing LoadBalancers", we really mean "testing cloud-provider-gcp", because as discussed in #36720, in kube-proxy load balancer services don't scale any differently than any other kind of services; it's only the cloud provider where we need the extra testing.) /sig scalability /sig network /sig cloud-provider /area provider/gcp cc @wojtek-t @aojea @bowei