partial fix https://github.com/flutter/flutter/issues/126826 (date range picker is another PR)
fixes https://github.com/flutter/flutter/issues/126597

### Description

1. This PR adds a bunch of M3 date picker tests
2. Fixes divider taking more space than it should
3. Added dividerColor  theme value to allow users to customise divider color just for the date pickers from date picker theme 

<details> 
<summary>code sample</summary> 

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

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        datePickerTheme: DatePickerThemeData(
          headerBackgroundColor: Colors.amber,
        ),
        useMaterial3: true,
      ),
      home: Scaffold(
        body: Builder(builder: (BuildContext context) {
          return Center(
            child: ElevatedButton(
              onPressed: () async {
                showDatePicker(
                  context: context,
                  initialDate: DateTime(2016, DateTime.january, 15),
                  firstDate: DateTime(2001),
                  lastDate: DateTime(2031, DateTime.december, 31),
                );
              },
              child: const Text('Show Date Picker'),
            ),
          );
        }),
      ),
    );
  }
}
``` 
	
</details>

### Before
![Screenshot 2023-05-19 at 17 32 19](https://github.com/flutter/flutter/assets/48603081/4463de1a-fb94-4930-a6ab-8245331a8134)

### After
![Screenshot 2023-05-19 at 17 51 15](https://github.com/flutter/flutter/assets/48603081/296276f0-cf13-4a59-8542-a46da774153b)