hello/reverse: add example example test

This will be linked from an updated go.dev/blog/examples.

For #61722.

Change-Id: I329141141e0590a6d7ee0b6b504c8f7bdc9c2d5f
Reviewed-on: https://go-review.googlesource.com/c/example/+/515237
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

Author: rsc

hello/reverse/example_test.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package reverse_test
+
+import (
+	"fmt"
+
+	"golang.org/x/example/hello/reverse"
+)
+
+func ExampleString() {
+	fmt.Println(reverse.String("hello"))
+	// Output: olleh
+}