lite2: actually run example tests + clock drift (#4487)

Closes: #4488

## Commits:

* run example tests

* introduce max clock drift

clockDrift variable from the spec.
10s should cover most of the clients.

References:

- http://vancouver-webpages.com/time/web.html
- https://blog.codinghorror.com/keeping-time-on-the-pc/

* fix ExampleClient_Update

* add test

* increase clock drift

* fix linter warning

lite2/example_test.go

@@ -11,7 +11,6 @@ import (
 	dbm "github.com/tendermint/tm-db"
 
 	"github.com/tendermint/tendermint/abci/example/kvstore"
-	"github.com/tendermint/tendermint/libs/log"
 	"github.com/tendermint/tendermint/lite2/provider"
 	httpp "github.com/tendermint/tendermint/lite2/provider/http"
 	dbs "github.com/tendermint/tendermint/lite2/store/db"
@@ -19,7 +18,7 @@ import (
 )
 
 // Automatically getting new headers and verifying them.
-func TestExample_Client_AutoUpdate(t *testing.T) {
+func ExampleClient_Update() {
 	// give Tendermint time to generate some blocks
 	time.Sleep(5 * time.Second)
 
@@ -57,10 +56,10 @@ func TestExample_Client_AutoUpdate(t *testing.T) {
 			Hash:   header.Hash(),
 		},
 		primary,
-		[]provider.Provider{primary}, // TODO: primary should not be used here
+		[]provider.Provider{primary}, // NOTE: primary should not be used here
 		dbs.New(db, chainID),
-		UpdatePeriod(1*time.Second),
-		Logger(log.TestingLogger()),
+		UpdatePeriod(0), // NOTE: value should be greater than zero
+		// Logger(log.TestingLogger()),
 	)
 	if err != nil {
 		stdlog.Fatal(err)
@@ -76,17 +75,29 @@ func TestExample_Client_AutoUpdate(t *testing.T) {
 
 	time.Sleep(2 * time.Second)
 
+	// XXX: 30 * time.Minute clock drift is needed because a) Tendermint strips
+	// monotonic component (see types/time/time.go) b) single instance is being
+	// run.
+	err = c.Update(time.Now().Add(30 * time.Minute))
+	if err != nil {
+		stdlog.Fatal(err)
+	}
+
 	h, err := c.TrustedHeader(0)
 	if err != nil {
 		stdlog.Fatal(err)
 	}
 
-	fmt.Println("got header", h.Height)
-	// Output: got header 3
+	if h.Height > 2 {
+		fmt.Println("successful update")
+	} else {
+		fmt.Println("update failed")
+	}
+	// Output: successful update
 }
 
 // Manually getting headers and verifying them.
-func TestExample_Client_ManualUpdate(t *testing.T) {
+func ExampleClient_VerifyHeaderAtHeight() {
 	// give Tendermint time to generate some blocks
 	time.Sleep(5 * time.Second)
 
@@ -127,17 +138,12 @@ func TestExample_Client_ManualUpdate(t *testing.T) {
 		[]provider.Provider{primary}, // TODO: primary should not be used here
 		dbs.New(db, chainID),
 		UpdatePeriod(0),
-		Logger(log.TestingLogger()),
+		// Logger(log.TestingLogger()),
 	)
 	if err != nil {
 		stdlog.Fatal(err)
 	}
-	err = c.Start()
-	if err != nil {
-		stdlog.Fatal(err)
-	}
 	defer func() {
-		c.Stop()
 		c.Cleanup()
 	}()
 
@@ -158,7 +164,7 @@ func TestExample_Client_ManualUpdate(t *testing.T) {
 func TestMain(m *testing.M) {
 	// start a tendermint node (and kvstore) in the background to test against
 	app := kvstore.NewApplication()
-	node := rpctest.StartTendermint(app)
+	node := rpctest.StartTendermint(app, rpctest.SuppressStdout)
 
 	code := m.Run()