kind jobs: stop relying on implicit SKIP=[Serial]

[pohly]

All jobs using e2e-k8s.sh with PARALLEL=true automatically skipped serial tests even though technically they now could run in the job: https://github.com/kubernetes-sigs/kind/blame/d1eecc46e30cac9d35cd32dc52677ef75ec22e18/hack/ci/e2e-k8s.sh#L226-L234

kubernetes-sigs/kind#4015 is changing that mandatory skip for jobs using LABEL_FILTER because it may be desirable to include serial tests, depending on the job. It's also better to be explicit about it in each job's LABEL_FILTER to avoid confusion and potential mistakes (not running tests that were expected to run).

To prepare for that change, jobs get updated based on the following principles:

• If a presubmit runs infrequently and is only invoked to test certain aspects, then including serial tests is desirable to get full test coverage of that aspect. Example: pull-kubernetes-e2e-kind-beta-features
• If a presubmit runs always, serial tests should be excluded. Example: pull-kubernetes-e2e-kind
• Periodic jobs should always run all supported tests, including the serial ones. Example: ci-kubernetes-e2e-kind
• Canary jobs match the behavior of the non-canary variant. Example: pull-kubernetes-e2e-kind-canary
• Jobs for release branches should better not change for the sake of preserving the old behavior (but that
  is debatable). Example: ci-kubernetes-e2e-kind-1-34

To make it more obvious where the upcoming e2e-k8s.sh will change test selection, "Includes serial tests for the sake of completeness." comments get added. Those are not true right now, but will be once the script is changed.

[k8s-ci-robot]

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[pohly]

/hold

Needs to be reviewed together with kubernetes-sigs/kind#4015. This PR then needs to be merged first.

[pohly]

/assign @BenTheElder

@dims: this changes some of your jobs. Do you agree that including serial tests makes sense for them?

@aojea: same with your network jobs. You expressed a desired to exclude serial tests from presubmits, so I leaned in that direction for them.

[pohly]

Maintaining two variants is more work and makes the testgrid dashboards larger.

I think that's done for jobs not using LABEL_FILTER yet. There is a ci-kubernetes-kube-network-policies-conformance-parallel but no corresponding ci-kubernetes-kube-network-policies-conformance-serial. With this change there is no need for it.

I was worried that that tests like test/e2e/apimachinery/garbage_collector.go: framework.ConformanceIt("should orphan pods created by rc if delete options say so", framework.WithSerial() might not have run as often as desired. But
ci-kubernetes-conformance-kind-ga-only runs everything sequentially, so at least serial conformance tests are covered as release blocking.

The same cannot be said for other tests like test/e2e/storage/testsuites/volumelimits.go: f.It("should support volume limits", f.WithSerial(). That's not in kind-master, nor is there a serial variant of it. It is in gce-cos-master-serial though as release informing.