refactor: fix lint issues and wire up main screen

- Use const Iperf3Result constructor in tests
- Remove unused import from widget_test.dart
- Fix unnecessary underscore warnings in Consumer builders
- Use initializing formal for iperf3Service field
- Wire up main.dart to call Iperf3BinaryPathProvider.initialize()

test/widget_test.dart

@@ -5,26 +5,13 @@
 // gestures. You can also use WidgetTester to find child widgets in the widget
 // tree, read text, and verify that the values of widget properties are correct.
 
-import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-import 'package:iperf3_gui/main.dart';
+import 'package:iperf3gui/main.dart';
 
 void main() {
-  testWidgets('Counter increments smoke test', (WidgetTester tester) async {
-    // Build our app and trigger a frame.
-    await tester.pumpWidget(const MyApp());
-
-    // Verify that our counter starts at 0.
-    expect(find.text('0'), findsOneWidget);
-    expect(find.text('1'), findsNothing);
-
-    // Tap the '+' icon and trigger a frame.
-    await tester.tap(find.byIcon(Icons.add));
-    await tester.pump();
-
-    // Verify that our counter has incremented.
-    expect(find.text('0'), findsNothing);
-    expect(find.text('1'), findsOneWidget);
+  testWidgets('App smoke test', (WidgetTester tester) async {
+    await tester.pumpWidget(const Iperf3GuiApp());
+    expect(find.text('iperf3-gui'), findsOneWidget);
   });
 }