e2e framework: clarify Alpha/Beta requirement for feature gates

We want:

- To keep test annotations simple, using both WithFeatureGate
  and WithFeature should only be necessary when a test really
  has requirements that go beyond "feature gate needs to be enabled".

- To run tests which depend only on feature gates being enabled
  in the ci-kubernetes-e2e-kind-alpha-features resp.
  ci-kubernetes-e2e-kind-beta-features, because otherwise we
  may have a proliferation of many bespoke jobs which only run
  very few tests. This would make testing more expensive for
  Kubernetes.

- To enable those tests only once in the ci-kubernetes-e2e-kind-alpha-features
  and ci-kubernetes-e2e-kind-beta-features definition instead
  of having to update those each time feature gates change.

This can be achieved by adding `Feature:Alpha` resp. `Feature:Beta` as Ginkgo
labels instead of just `Alpha` and `Beta`. Then jobs which are configured to
skip tests with feature dependencies via --label-filter=!/Feature:.+/ will skip
tests which are labeled with just WithFeatureGate. The ci-kubernetes jobs
can select to include such tests with a special regexp that mimicks
a negative lookahead (see k8s.io/community/contributors/devel/sig-testing/e2e-tests.md)

Note that removing WithFeature depends on first updating job definitions to use
--label-filter or to skip based on the inline `[Alpha]` or `[Beta]` text,
otherwise tests that were previously skipped because of WithFeature might
start to run in jobs which don't have the feature gate enabled.

test/e2e/framework/ginkgowrapper.go

@@ -209,8 +209,9 @@ func registerInSuite(ginkgoCall func(string, ...interface{}) bool, args []interf
 		case label:
 			fullLabel := strings.Join(arg.parts, ":")
 			addLabel(fullLabel)
-			if arg.extra != "" {
+			if arg.extraFeature != "" {
-				addLabel(arg.extra)
+				texts = append(texts, fmt.Sprintf("[%s]", arg.extraFeature))
+				ginkgoArgs = append(ginkgoArgs, ginkgo.Label("Feature:"+arg.extraFeature))
 			}
 			if fullLabel == "Serial" {
 				ginkgoArgs = append(ginkgoArgs, ginkgo.Serial)
@@ -309,6 +310,10 @@ func validateText(location types.CodeLocation, text string, labels []string) {
 			recordTextBug(location, fmt.Sprintf("[%s] in plain text is deprecated and must be added through With%s instead", tag, tag))
 		}
 		if deprecatedStability.Has(tag) {
+			if slices.Contains(labels, "Feature:"+tag) {
+				// Okay, was also set as label.
+				continue
+			}
 			recordTextBug(location, fmt.Sprintf("[%s] in plain text is deprecated and must be added by defining the feature gate through WithFeatureGate instead", tag))
 		}
 		if index := strings.Index(tag, ":"); index > 0 {
@@ -353,6 +358,16 @@ func withFeature(name Feature) interface{} {
 // [k8s.io/apiserver/pkg/util/feature.DefaultMutableFeatureGate]. Once a
 // feature gate gets removed from there, the WithFeatureGate calls using it
 // also need to be removed.
+//
+// [Alpha] resp. [Beta] get added to the test name automatically depending
+// on the current stability level of the feature. Feature:Alpha resp.
+// Feature:Beta get added to the Ginkgo labels because this is a special
+// requirement for how the cluster needs to be configured.
+//
+// If the test can run in any cluster that has alpha resp. beta features and
+// API groups enabled, then annotating it with just WithFeatureGate is
+// sufficient. Otherwise, WithFeature has to be used to define the additional
+// requirements.
 func WithFeatureGate(featureGate featuregate.Feature) interface{} {
 	return withFeatureGate(featureGate)
 }
@@ -376,7 +391,7 @@ func withFeatureGate(featureGate featuregate.Feature) interface{} {
 	}
 
 	l := newLabel("FeatureGate", string(featureGate))
-	l.extra = level
+	l.extraFeature = level
 	return l
 }
 
@@ -544,8 +559,9 @@ func withFlaky() interface{} {
 type label struct {
 	// parts get concatenated with ":" to build the full label.
 	parts []string
-	// extra is an optional fully-formed extra label.
+	// extra is an optional feature name. It gets added as [<extraFeature>]
-	extra string
+	// to the test name and as Feature:<extraFeature> to the labels.
+	extraFeature string
 	// explanation gets set for each label to help developers
 	// who pass a label to a ginkgo function. They need to use
 	// the corresponding framework function instead.
@@ -572,7 +588,7 @@ func TagsEqual(a, b interface{}) bool {
 	if !ok {
 		return false
 	}
-	if al.extra != bl.extra {
+	if al.extraFeature != bl.extraFeature {
 		return false
 	}
 	return slices.Equal(al.parts, bl.parts)

test/e2e/framework/internal/unittests/bugs/bugs.go

@@ -133,12 +133,12 @@ ERROR: some/relative/path/buggy.go:200: with spaces
 
 	// Used by unittests/list-labels.
 	ListLabelsOutput = `The following labels can be used with 'ginkgo run --label-filter':
-    Alpha
-    Beta
     Conformance
     Disruptive
     Environment:Linux
     Environment:no-such-env
+    Feature:Alpha
+    Feature:Beta
     Feature:feature-foo
     Feature:no-such-feature
     FeatureGate:TestAlphaFeature

test/e2e/framework/internal/unittests/list-labels/listlabels_test.go

@@ -26,7 +26,7 @@ import (
 	"k8s.io/kubernetes/test/e2e/framework/internal/unittests/bugs"
 )
 
-func TestListTests(t *testing.T) {
+func TestListLabels(t *testing.T) {
 	bugs.Describe()
 	framework.CheckForBugs = false
 	output, code := unittests.GetFrameworkOutput(t, map[string]string{"list-labels": "true"})