Deflake: TestFirstCommitNotification

Infrequently the test flaked. Reproducable with:

```
  go test go.etcd.io/etcd/tests/v3/integration --run TestFirstCommitNotification --count=500
```

The moveLeader finishes when configchange is commited by quorum.
It doesn't guarantee that the 'empty' record was committed by the new leader.
From time to time happened that appliedLeaderIndex was returning 9
(without empty entry) and the test flaked. In healthy case the
appliedIndex returned 10.

Fixed by putting kv pair after leader change. The pair is guaranteed
to be stored on index when put finishes (so the empty entry as well).

tests/integration/v3_leadership_test.go

@@ -184,6 +184,7 @@ func TestTransferLeadershipWithLearner(t *testing.T) {
 
 func TestFirstCommitNotification(t *testing.T) {
 	BeforeTest(t)
+	ctx := context.Background()
 	clusterSize := 3
 	cluster := NewClusterV3(t, &ClusterConfig{Size: clusterSize})
 	defer cluster.Terminate(t)
@@ -205,6 +206,15 @@ func TestFirstCommitNotification(t *testing.T) {
 		t.Errorf("got error during leadership transfer: %v", err)
 	}
 
+	t.Logf("Leadership transferred.")
+	t.Logf("Submitting write to make sure empty and 'foo' index entry was already flushed")
+	cli := cluster.RandClient()
+
+	if _, err := cli.Put(ctx, "foo", "bar"); err != nil {
+		t.Fatalf("Failed to put kv pair.")
+	}
+
+	// It's guaranteed now that leader contains the 'foo'->'bar' index entry.
 	leaderAppliedIndex := cluster.Members[newLeaderIdx].s.AppliedIndex()
 
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
@@ -215,7 +225,7 @@ func TestFirstCommitNotification(t *testing.T) {
 	for i, notifier := range notifiers {
 		member, notifier := cluster.Members[i], notifier
 		group.Go(func() error {
-			return checkFirstCommitNotification(groupContext, member, leaderAppliedIndex, notifier)
+			return checkFirstCommitNotification(t, groupContext, member, leaderAppliedIndex, notifier)
 		})
 	}
 
@@ -226,21 +236,22 @@ func TestFirstCommitNotification(t *testing.T) {
 }
 
 func checkFirstCommitNotification(
+	t testing.TB,
 	ctx context.Context,
 	member *member,
 	leaderAppliedIndex uint64,
 	notifier <-chan struct{},
 ) error {
 	// wait until server applies all the changes of leader
 	for member.s.AppliedIndex() < leaderAppliedIndex {
+		t.Logf("member.s.AppliedIndex():%v <= leaderAppliedIndex:%v", member.s.AppliedIndex(), leaderAppliedIndex)
 		select {
 		case <-ctx.Done():
 			return ctx.Err()
 		default:
 			time.Sleep(100 * time.Millisecond)
 		}
 	}
-
 	select {
 	case msg, ok := <-notifier:
 		if ok {
@@ -251,6 +262,7 @@ func checkFirstCommitNotification(
 			)
 		}
 	default:
+		t.Logf("member.s.AppliedIndex():%v >= leaderAppliedIndex:%v", member.s.AppliedIndex(), leaderAppliedIndex)
 		return fmt.Errorf(
 			"notification was not triggered, member ID: %d",
 			member.ID(),