io/ioutil: modify example test for WriteFile to allow it to run in the playground

Author: pantonshire

src/io/ioutil/example_test.go

@@ -125,6 +125,11 @@ func ExampleReadFile() {
 
 func ExampleWriteFile() {
 	message := []byte("Hello, Gophers!")
+
+	if err := os.MkdirAll("testdata", 0755); err != nil {
+		log.Fatal(err)
+	}
+
 	err := ioutil.WriteFile("testdata/hello", message, 0644)
 	if err != nil {
 		log.Fatal(err)