[cupertino_icons] Update code excerpts and add doc-level comment in t…

…est file

third_party/packages/cupertino_icons/example/example.md

@@ -1,14 +1,13 @@
 <?code-excerpt path-base="../test"?>
 
-Please find the usage of [`cupertino_icons`](https://pub.dev/packages/cupertino_icons) which is used via Flutter's `CupertinoIcons` class below:
+This package is used via [`CupertinoIcons`](https://api.flutter.dev/flutter/cupertino/CupertinoIcons-class.html):
 
 <?code-excerpt "cupertino_icons_test.dart (CupertinoIcon)"?>
 ```dart
 const Icon icon = Icon(
   CupertinoIcons.heart_fill,
   color: Colors.pink,
   size: 24.0,
-  semanticLabel: 'Text to announce in accessibility modes',
 );
 ```
 

third_party/packages/cupertino_icons/test/cupertino_icons_test.dart

@@ -1,6 +1,7 @@
 // Copyright 2013 The Flutter Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
+/// This test file is primarily here to serve as a source for code excerpts.
 
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
@@ -15,7 +16,6 @@ void main() {
         CupertinoIcons.heart_fill,
         color: Colors.pink,
         size: 24.0,
-        semanticLabel: 'Text to announce in accessibility modes',
       );
       // #enddocregion CupertinoIcon