make ExampleExecAllocatorOption less flaky

The Default folder is created asynchronously to Chrome starting to
listen on the debugging protocol port. So we can't expect it to exist.

Instead, base the example on DevToolsActivePort, which we can rely on.

example_test.go

@@ -1,10 +1,12 @@
 package chromedp_test
 
 import (
+	"bytes"
 	"context"
 	"fmt"
 	"io/ioutil"
 	"os"
+	"path/filepath"
 
 	"github.com/chromedp/chromedp"
 )
@@ -59,21 +61,18 @@ func ExampleExecAllocatorOption() {
 		panic(err)
 	}
 
-	files, err := ioutil.ReadDir(dir)
+	path := filepath.Join(dir, "DevToolsActivePort")
+	bs, err := ioutil.ReadFile(path)
 	if err != nil {
 		panic(err)
 	}
-	fmt.Println("Files in UserDataDir:")
-	for _, file := range files {
-		fmt.Println(file.Name())
-	}
+	lines := bytes.Split(bs, []byte("\n"))
+	fmt.Printf("DevToolsActivePort has %d lines\n", len(lines))
 
 	// wait for the resources to be cleaned up
 	cancel()
 	chromedp.FromContext(allocCtx).Allocator.Wait()
 
 	// Output:
-	// Files in UserDataDir:
-	// Default
-	// DevToolsActivePort
+	// DevToolsActivePort has 2 lines
 }