speed up the screenshot tests, and test the images

Using a smaller viewport speeds up both tests, and lets us know what
dimensions to expect in TestCaptureScreenshot.

For TestScreenshot, we can know what dimensions to expect in advance, as
we have the images in testdata.

'go test -run Screenshot' goes from ~0.9s to ~0.5s on my machine.

Finally, don't run ExampleTitle as part of 'go test', as it's slow.

example_test.go

@@ -30,8 +30,8 @@ func ExampleTitle() {
 	cancel()
 	chromedp.FromContext(ctx).Allocator.Wait()
 
-	// Output:
-	// Issues · chromedp/chromedp · GitHub
+	// no expected output, to not run this test as part of 'go test'; it's
+	// too slow, requiring internet access.
 }
 
 func ExampleExecAllocatorOption() {

nav_test.go

@@ -1,10 +1,14 @@
 package chromedp
 
 import (
+	"bytes"
+	"image"
+	_ "image/png"
 	"strings"
 	"testing"
 	"time"
 
+	"github.com/chromedp/cdproto/emulation"
 	"github.com/chromedp/cdproto/page"
 )
 
@@ -213,18 +217,28 @@ func TestCaptureScreenshot(t *testing.T) {
 	ctx, cancel := testAllocate(t, "image.html")
 	defer cancel()
 
+	// set the viewport size, to know what screenshot size to expect
+	width, height := 650, 450
 	var buf []byte
 	if err := Run(ctx, Tasks{
+		emulation.SetDeviceMetricsOverride(int64(width), int64(height), 1.0, false),
 		WaitVisible(`#icon-brankas`, ByID), // for image.html
 		CaptureScreenshot(&buf),
 	}); err != nil {
 		t.Fatal(err)
 	}
 
-	if len(buf) == 0 {
-		t.Fatal("failed to capture screenshot")
+	config, format, err := image.DecodeConfig(bytes.NewReader(buf))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if want := "png"; format != want {
+		t.Fatalf("expected format to be %q, got %q", want, format)
+	}
+	if config.Width != width || config.Height != height {
+		t.Fatalf("expected dimensions to be %d*%d, got %d*%d",
+			width, height, config.Width, config.Height)
 	}
-	//TODO: test image
 }
 
 /*func TestAddOnLoadScript(t *testing.T) {

query_test.go

@@ -1,7 +1,10 @@
 package chromedp
 
 import (
+	"bytes"
 	"fmt"
+	"image"
+	_ "image/png"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
@@ -12,6 +15,7 @@ import (
 	"github.com/chromedp/cdproto/cdp"
 	"github.com/chromedp/cdproto/css"
 	"github.com/chromedp/cdproto/dom"
+	"github.com/chromedp/cdproto/emulation"
 
 	"github.com/chromedp/chromedp/kb"
 )
@@ -739,13 +743,21 @@ func TestScreenshot(t *testing.T) {
 	defer cancel()
 
 	tests := []struct {
-		sel string
-		by  QueryOption
+		sel  string
+		by   QueryOption
+		size int
 	}{
-		{"/html/body/img", BySearch},
-		{"img", ByQueryAll},
-		{"img", ByQuery},
-		{"#icon-github", ByID},
+		{"/html/body/img", BySearch, 239},
+		{"img", ByQueryAll, 239},
+		{"#icon-github", ByID, 120},
+	}
+
+	// a smaller viewport speeds up this test
+	width, height := 650, 450
+	if err := Run(ctx, emulation.SetDeviceMetricsOverride(
+		int64(width), int64(height), 1.0, false,
+	)); err != nil {
+		t.Fatal(err)
 	}
 
 	for i, test := range tests {
@@ -757,7 +769,17 @@ func TestScreenshot(t *testing.T) {
 		if len(buf) == 0 {
 			t.Fatalf("test %d failed to capture screenshot", i)
 		}
-		//TODO: test image
+		config, format, err := image.DecodeConfig(bytes.NewReader(buf))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if want := "png"; format != want {
+			t.Fatalf("expected format to be %q, got %q", want, format)
+		}
+		if config.Width != test.size || config.Height != test.size {
+			t.Fatalf("expected dimensions to be %d*%d, got %d*%d",
+				test.size, test.size, config.Width, config.Height)
+		}
 	}
 }