add timer widget test

test/widget_tests/timer_widget_test.dart

@@ -0,0 +1,32 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider/provider.dart';
+import 'package:smoke_cess_app/providers/timer_provider.dart';
+import 'package:smoke_cess_app/utils/timer_util.dart';
+import 'package:smoke_cess_app/widgets/timer_widget.dart';
+
+void main() {
+  group('TimerWidget', () {
+    testWidgets('should display duration', (WidgetTester tester) async {
+      TimerProvider timerProvider = TimerProvider();
+      Duration duration = const Duration(minutes: 1);
+      await tester.pumpWidget(
+        MaterialApp(
+            home: Scaffold(
+                body: ChangeNotifierProvider(
+                    create: (context) => timerProvider,
+                    child: TimerWidget(duration: duration)))),
+      );
+
+      final durationTextFinder = find.text(formatTime(duration.inSeconds));
+      expect(durationTextFinder, findsOneWidget);
+
+      timerProvider.startTimer(duration);
+      await tester.pump(const Duration(seconds: 1));
+      expect(durationTextFinder, findsNothing);
+      expect(find.text(formatTime(duration.inSeconds - 1)), findsOneWidget);
+    });
+  });
+}