Add onstage offstage support

[rehlma]

By default, spot() only finds widgets that are "onstage", not hidden with the Offstage widget.

To find offstage widgets, start your widget selector with spotOffstage(). Search for both - the on- and offstage widgets - with spotAllWidgets().

For existing selectors, use overrideWidgetPresence(WidgetPresence presence) to modify the presence to offstage, onstage or combined.

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:spot/spot.dart';

void main() {
  testWidgets('Spot offstage and combined widgets', (tester) async {
    await tester.pumpWidget(
      MaterialApp(
        home: Row(
          children: [
            Text('a'),
            Text('b'),
            Offstage(child: Text('c')),
          ],
        ),
      ),
    );

    spot<Text>().withText('a').existsOnce();
    spot<Text>().withText('c').doesNotExist();
    spot<Text>().withText('c').overrideWidgetPresence(WidgetPresence.offstage).existsOnce();

    spotOffstage().spot<Text>().atMost(3);
    spotOffstage().spotText('c').existsOnce();
    spotOffstage().overrideWidgetPresence(WidgetPresence.onstage).spotText('c').doesNotExist();

    spotAllWidgets().spotText('a').existsOnce();
    spotAllWidgets().spotText('c').existsOnce();
    spotOffstage().overrideWidgetPresence(WidgetPresence.combined).spotText('a').existsOnce();
    spotOffstage().overrideWidgetPresence(WidgetPresence.combined).spotText('c').existsOnce();
  });
}