Add test for dismissible.0.dart API example. (#146723)

This PR contributes to https://github.com/flutter/flutter/issues/130459

### Description
- Adds `examples/api/test/widgets/dismissible/dismissible.0_test.dart` test

dev/bots/check_code_samples.dart

@@ -477,7 +477,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/implicit_animations/animated_positioned.0_test.dart',
   'examples/api/test/widgets/implicit_animations/animated_padding.0_test.dart',
   'examples/api/test/widgets/implicit_animations/sliver_animated_opacity.0_test.dart',
-  'examples/api/test/widgets/dismissible/dismissible.0_test.dart',
   'examples/api/test/widgets/scroll_view/custom_scroll_view.1_test.dart',
   'examples/api/test/widgets/inherited_notifier/inherited_notifier.0_test.dart',
   'examples/api/test/animation/curves/curve2_d.0_test.dart',

examples/api/test/widgets/dismissible/dismissible.0_test.dart

@@ -0,0 +1,81 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/dismissible/dismissible.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Future<void> dismissHorizontally({
+    required WidgetTester tester,
+    required Finder finder,
+    required AxisDirection direction,
+  }) async {
+    final double width = (tester.renderObject(finder) as RenderBox).size.width;
+    final double dx = width * 0.8;
+
+    final Offset offset = switch (direction) {
+      AxisDirection.left => Offset(-dx, 0.0),
+      AxisDirection.right => Offset(dx, 0.0),
+      _ => throw ArgumentError('$direction is not supported'),
+    };
+
+    await tester.drag(finder, offset);
+  }
+
+  testWidgets(
+    'ListTiles can be dismissed from right to left',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.DismissibleExampleApp(),
+      );
+
+      for (final int index in <int>[0, 33, 66, 99]) {
+        final ValueKey<int> key = ValueKey<int>(index);
+
+        await tester.scrollUntilVisible(find.byKey(key), 100);
+
+        expect(find.byKey(key), findsOneWidget);
+
+        await dismissHorizontally(
+          tester: tester,
+          finder: find.byKey(key),
+          direction: AxisDirection.left,
+        );
+
+        await tester.pumpAndSettle();
+
+        expect(find.byKey(key), findsNothing);
+      }
+    },
+  );
+
+  testWidgets(
+    'ListTiles can be dismissed from left to right',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const example.DismissibleExampleApp(),
+      );
+
+      for (final int index in <int>[0, 33, 66, 99]) {
+        final ValueKey<int> key = ValueKey<int>(index);
+
+        await tester.scrollUntilVisible(find.byKey(key), 100);
+
+        expect(find.byKey(key), findsOneWidget);
+
+        await dismissHorizontally(
+          tester: tester,
+          finder: find.byKey(key),
+          direction: AxisDirection.right,
+        );
+
+        await tester.pumpAndSettle();
+
+        expect(find.byKey(key), findsNothing);
+      }
+    },
+  );
+}