Flutter / Dart: onPressed not called when tapping an AppBar widget in Widget Test

my problem is that I have a stateless widget that returns a AppBar with an onPressed event. In my corresponding widgetTest, I tap the component and expect that the onPressed method is called. However, it is not.

This is my widget:

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

class IconAppBar extends StatelessWidget with PreferredSizeWidget {
  final VoidCallback onPressed;

  IconAppBar({this.onPressed});

  @override
  Widget build(BuildContext context) {
    return AppBar(
      leading: IconButton(
        color: Colors.black,
        icon: SvgPicture.asset(
          "resources/images/icon.svg",
          width: 24,
          height: 24,
        ),
        onPressed: onPressed,
      ),
    );
  }

  @override
  Size get preferredSize => Size.fromHeight(kToolbarHeight);
}

In the test, I create the widget with a simple test for the onPressed if the component was tapped.

void main() {
    testWidgets("Should run onPressed when tapped",
        (WidgetTester tester) async {
      var counter = 0;
      void callback() {
        counter++;
      }

      await tester.pumpWidget(
        MediaQuery(
          data: MediaQueryData(),
          child: MaterialApp(
            localizationsDelegates: [
              GlobalMaterialLocalizations.delegate,
              GlobalWidgetsLocalizations.delegate,
              GlobalCupertinoLocalizations.delegate,
            ],
            home: IconAppBar(
              onPressed: callback,
            ),
          ),
        ),
      );

      await tester.pump(Duration(milliseconds: 300));
      await tester.tap(find.byType(IconAppBar));
      await tester.pump(Duration(milliseconds: 300));

      expect(counter, 1); // fails because actual counter is 0
    });
}

Changing to tester.pump() or tester.pumpAndSettle() did not change anything. The method is not called.

Edit: Running this on an actual device or in an emulator, the onPressed works as expected.

I would really appreciate any help or ideas. Thanks!