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.

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)
+		}
 	}
 }