dev_dependencies:
  flutter_test:
    sdk: flutter

import 'package:flutter/material.dart';

class CounterPage extends StatefulWidget {
  @override
  _CounterPageState createState() => _CounterPageState();
}

class _CounterPageState extends State<CounterPage> {
  int count = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('นับจำนวน')),
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text('คุณกดแล้ว:', key: Key('label')),
          Text('$count', key: Key('counter'), style: TextStyle(fontSize: 32)),
          ElevatedButton(
            key: Key('increment'),
            onPressed: () => setState(() => count++),
            child: Text('เพิ่ม'),
          ),
        ],
      ),
    );
  }
}

import 'package:flutter_test/flutter_test.dart';
import 'package:flutter/material.dart';
import 'counter_page.dart';

void main() {
  testWidgets('กดปุ่มแล้วค่าเพิ่ม', (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(home: CounterPage()));

    expect(find.text('0'), findsOneWidget);
    await tester.tap(find.byKey(Key('increment')));
    await tester.pump();
    expect(find.text('1'), findsOneWidget);
  });
}

flutter test test/counter_test.dart