fix analysis issue of using deprecated mock method call handler sets

Author: julian-CStack

test/stack_wallet_backup_method_channel_test.dart

@@ -9,13 +9,19 @@ void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   setUp(() {
-    channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      return '42';
-    });
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(
+      channel,
+      (methodCall) async => '42',
+    );
   });
 
   tearDown(() {
-    channel.setMockMethodCallHandler(null);
+    TestDefaultBinaryMessengerBinding.instance.defaultBinaryMessenger
+        .setMockMethodCallHandler(
+      channel,
+      (methodCall) => null,
+    );
   });
 
   test('getPlatformVersion', () async {