add basic testing for channel_preview widget

test/src/channel_preview_test.dart

@@ -0,0 +1,52 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:mockito/mockito.dart';
+import 'package:stream_chat_flutter/stream_chat_flutter.dart';
+
+class MockClient extends Mock implements Client {}
+
+class MockChannel extends Mock implements Channel {}
+
+class MockChannelState extends Mock implements ChannelClientState {}
+
+void main() {
+  testWidgets(
+    'it should show basic channel information',
+    (WidgetTester tester) async {
+      final client = MockClient();
+      final channel = MockChannel();
+      final channelState = MockChannelState();
+      final lastMessageAt = DateTime.parse('2020-06-22 12:00:00');
+
+      when(channel.lastMessageAt).thenReturn(lastMessageAt);
+      when(channel.state).thenReturn(channelState);
+      when(channel.extraData).thenReturn({
+        'name': 'test name',
+      });
+      when(channelState.unreadCount).thenReturn(1);
+      when(channelState.lastMessage).thenReturn(Message(
+        text: 'hello',
+      ));
+
+      await tester.pumpWidget(MaterialApp(
+        home: StreamChat(
+          client: client,
+          child: StreamChannel(
+            channel: channel,
+            child: Scaffold(
+              body: ChannelPreview(
+                channel: channel,
+              ),
+            ),
+          ),
+        ),
+      ));
+
+      expect(find.text('22/06/2020'), findsOneWidget);
+      expect(find.text('test name'), findsOneWidget);
+      expect(find.text('1'), findsOneWidget);
+      expect(find.text('hello'), findsOneWidget);
+      expect(find.byType(ChannelImage), findsOneWidget);
+    },
+  );
+}