fixes [[Proposal] Improve TabBarTheme styling API for indicator color ](https://github.com/flutter/flutter/issues/130392)

### Description

This fixes an issue where the `TabBarTheme.indicator` isn't applied in Material 2 and also adds indicator color tests for both M3 and M2.

### Code sample

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

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

/// Flutter code sample for [TabBar].

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        useMaterial3: false,
        tabBarTheme: const TabBarTheme(
          indicatorColor: Colors.amber,
        )
      ),
      home: const TabBarExample(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return DefaultTabController(
      initialIndex: 1,
      length: 3,
      child: Scaffold(
        appBar: AppBar(
          title: const Text('TabBar Sample'),
          bottom: const TabBar(
            tabs: <Widget>[
              Tab(
                icon: Icon(Icons.cloud_outlined),
                text: 'Cloudy',
              ),
              Tab(
                icon: Icon(Icons.beach_access_sharp),
                text: 'Sunny',
              ),
              Tab(
                icon: Icon(Icons.brightness_5_sharp),
                text: 'Rainy',
              ),
            ],
          ),
        ),
        body: const TabBarView(
          children: <Widget>[
            Center(
              child: Text("It's cloudy here"),
            ),
            Center(
              child: Text("It's rainy here"),
            ),
            Center(
              child: Text("It's sunny here"),
            ),
          ],
        ),
      ),
    );
  }
}
``` 
	
</details>

### Before
![Screenshot 2023-08-08 at 15 39 24](https://github.com/flutter/flutter/assets/48603081/9030e025-8615-45d0-a337-87ba2fdf6ca3)

### After
![Screenshot 2023-08-08 at 15 39 07](https://github.com/flutter/flutter/assets/48603081/4b98ac55-2d51-4a4c-93ba-7d36dc4be1d8)