support async call in testGoldens

[zhujiageng]

import 'package:flutter_test/flutter_test.dart';
import 'package:golden_toolkit/golden_toolkit.dart';

void main() {
  test('async call success', () async {
    await Future(() {
      print('async called');
    });
  });

  testGoldens('async call janked', (tester) async {
    await Future(() {
      print('async will never be called');
    });
  });
}

[fatherOfLegends]

This likely is not related to this package and is likely by design. testGoldens uses testWidgets internally not simply "test()" from dart.