L2 E2E tests: Flake fix: metrics validation of wrong speaker

In this test sometimes we get the wrong node when looking
for the advertising node of a service.
A newly created service can get the same IP the service from
the previous test got (since it was already deleted).
While the previous service is down it can take time for the
running host ARP table to be updated.

The solution for such situation is to look for the advertising
node of a service inside an eventually.

Fixing issue #1195.

An example of the failing test can be found here:
https://github.com/metallb/metallb/runs/4911555260?check_suite_focus=true

e2etest/l2tests/l2.go

@@ -428,13 +428,21 @@ var _ = ginkgo.Describe("L2", func() {
 			allNodes, err := cs.CoreV1().Nodes().List(context.Background(), metav1.ListOptions{})
 			framework.ExpectNoError(err)
 
-			advNode, err := advertisingNodeFromMAC(allNodes.Items, ingressIP, executor.Host)
-			framework.ExpectNoError(err)
-			advSpeaker, ok := speakerPods[advNode.Name]
-			framework.ExpectEqual(ok, true, fmt.Sprintf("could not find speaker pod on announcing node %s", advNode.Name))
-			delete(speakerPods, advSpeaker.Spec.NodeName)
-
+			var advNode *corev1.Node
+			var advSpeaker *corev1.Pod
 			gomega.Eventually(func() error {
+				var ok bool
+
+				advNode, err = advertisingNodeFromMAC(allNodes.Items, ingressIP, executor.Host)
+				if err != nil {
+					return err
+				}
+
+				advSpeaker, ok = speakerPods[advNode.Name]
+				if !ok {
+					return fmt.Errorf("could not find speaker pod on announcing node %s", advNode.Name)
+				}
+
 				speakerMetrics, err := metrics.ForPod(controllerPod, advSpeaker, metallb.Namespace)
 				if err != nil {
 					return err
@@ -464,6 +472,8 @@ var _ = ginkgo.Describe("L2", func() {
 			}, 2*time.Minute, 1*time.Second).Should(gomega.BeNil())
 
 			// Negative - validate that the other speakers don't publish layer2 metrics
+			delete(speakerPods, advSpeaker.Spec.NodeName)
+
 			for _, p := range speakerPods {
 				speakerMetrics, err := metrics.ForPod(controllerPod, p, metallb.Namespace)
 				framework.ExpectNoError(err)