Add `PopupMenuButton.iconColor`, `PopupMenuTheme.iconSize` and fix button icon using unexpected color propert (#132054)

fixes [PopupMenuButton uses color property for icon color](https://github.com/flutter/flutter/issues/127802) 
fixes [`popup_menu_test.dart` lacks default icon color tests.](https://github.com/flutter/flutter/issues/132050) 

### Description
- Add  `PopupMenuButton..iconColor` and fix the PopupMenu button icon using an unexpected color property.
- Add the missing `PopupMenuTheme.iconSize`.
- Clean up some tests and minor improvements.

### Code sample

<details> 
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

/// Flutter code sample for [PopupMenuButton].

// This is the type used by the popup menu below.
enum SampleItem { itemOne, itemTwo, itemThree }

void main() => runApp(const PopupMenuApp());

class PopupMenuApp extends StatelessWidget {
  const PopupMenuApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        popupMenuTheme: PopupMenuThemeData(
          // iconSize: 75,
          // iconColor: Colors.amber,
          color: Colors.deepPurple[100],
        ),
      ),
      home: const PopupMenuExample(),
    );
  }
}

class PopupMenuExample extends StatefulWidget {
  const PopupMenuExample({super.key});

  @override
  State<PopupMenuExample> createState() => _PopupMenuExampleState();
}

class _PopupMenuExampleState extends State<PopupMenuExample> {
  SampleItem? selectedMenu;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('PopupMenuButton')),
      body: Center(
        child: PopupMenuButton<SampleItem>(
          iconSize: 75,
          // iconColor: Colors.amber,
          color: Colors.deepPurple[100],
          initialValue: selectedMenu,
          // Callback that sets the selected popup menu item.
          onSelected: (SampleItem item) {
            setState(() {
              selectedMenu = item;
            });
          },
          itemBuilder: (BuildContext context) => <PopupMenuEntry<SampleItem>>[
            const PopupMenuItem<SampleItem>(
              value: SampleItem.itemOne,
              child: Text('Item 1'),
            ),
            const PopupMenuItem<SampleItem>(
              value: SampleItem.itemTwo,
              child: Text('Item 2'),
            ),
            const PopupMenuDivider(),
            const CheckedPopupMenuItem<SampleItem>(
              value: SampleItem.itemThree,
              checked: true,
              child: Text('Item 3'),
            ),
          ],
        ),
      ),
    );
  }
}

``` 
	
</details>

![Group 2](https://github.com/flutter/flutter/assets/48603081/eb5404ae-2a07-4374-9821-66a0bbea041e)

![Group 1](https://github.com/flutter/flutter/assets/48603081/464e3957-1afb-4118-abcc-aad12591dc51)