test: drive the real UI against an in-memory runner

FakeIperf3Runner implements Iperf3Runner, so the widget tests exercise the
actual widget tree — GO, STOP, the status line, the stat row, validation —
with no MethodChannel and no native library. That was impossible while the
screen constructed its own Iperf3Service.

The view model tests share the same fake instead of each file carrying a
double that had to mirror the concrete service.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/support/fake_iperf3_runner.dart

@@ -0,0 +1,62 @@
+import 'dart:async';
+
+import 'package:iperf3gui/domain/models/iperf3_event.dart';
+import 'package:iperf3gui/domain/models/iperf3_result.dart';
+import 'package:iperf3gui/domain/models/test_configuration.dart';
+import 'package:iperf3gui/domain/services/iperf3_runner.dart';
+
+/// In-memory [Iperf3Runner] for tests.
+///
+/// Implements the interface rather than the concrete service, so it tracks the
+/// five members the UI actually depends on instead of every public member the
+/// MethodChannel implementation happens to have.
+class FakeIperf3Runner implements Iperf3Runner {
+  FakeIperf3Runner({this.onRun});
+
+  /// Supplies the result for each [run], and may [emit] events first.
+  Future<Iperf3Result> Function(TestConfiguration)? onRun;
+
+  final StreamController<Iperf3Event> _controller =
+      StreamController<Iperf3Event>.broadcast();
+
+  /// Configurations passed to [run], in order.
+  final List<TestConfiguration> runCalls = [];
+
+  int stopCallCount = 0;
+  bool disposed = false;
+  bool _isRunning = false;
+
+  int get runCallCount => runCalls.length;
+
+  /// Push an event as if the native layer had reported it.
+  void emit(Iperf3Event event) => _controller.add(event);
+
+  @override
+  Stream<Iperf3Event> get events => _controller.stream;
+
+  @override
+  bool get isRunning => _isRunning;
+
+  @override
+  Future<Iperf3Result> run(TestConfiguration config) async {
+    runCalls.add(config);
+    _isRunning = true;
+    try {
+      if (onRun != null) return await onRun!(config);
+      return const Iperf3Result(rawOutput: '', exitCode: 0);
+    } finally {
+      _isRunning = false;
+    }
+  }
+
+  @override
+  Future<void> stop() async {
+    stopCallCount++;
+  }
+
+  @override
+  Future<void> dispose() async {
+    disposed = true;
+    await _controller.close();
+  }
+}