Add test for draggable.0.dart API example. (#147941)

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

### Description
- Adds test for `examples/api/lib/widgets/drag_target/draggable.0.dart`

dev/bots/check_code_samples.dart

@@ -396,7 +396,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/widgets/inherited_theme/inherited_theme.0_test.dart',
   'examples/api/test/widgets/sliver/decorated_sliver.0_test.dart',
   'examples/api/test/widgets/autofill/autofill_group.0_test.dart',
-  'examples/api/test/widgets/drag_target/draggable.0_test.dart',
   'examples/api/test/widgets/shared_app_data/shared_app_data.1_test.dart',
   'examples/api/test/widgets/shared_app_data/shared_app_data.0_test.dart',
   'examples/api/test/widgets/nested_scroll_view/nested_scroll_view_state.0_test.dart',

examples/api/test/widgets/drag_target/draggable.0_test.dart

@@ -0,0 +1,122 @@
+// 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/drag_target/draggable.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Finder findContainerWith({
+    required Finder child,
+    required Color color,
+  }) {
+    return find.ancestor(
+      of: child,
+      matching: find.byWidgetPredicate(
+        (Widget widget) => widget is Container && widget.color == color,
+      ),
+    );
+  }
+
+  testWidgets('Verify initial state', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.DraggableExampleApp(),
+    );
+
+    expect(find.text('Draggable Sample'), findsOneWidget);
+
+    expect(
+      findContainerWith(
+        color: Colors.lightGreenAccent,
+        child: find.text('Draggable'),
+      ),
+      findsOneWidget,
+    );
+
+    expect(
+      findContainerWith(
+        color: Colors.cyan,
+        child: find.text('Value is updated to: 0'),
+      ),
+      findsOneWidget,
+    );
+  });
+
+  testWidgets('Verify correct containers are displayed while dragging', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.DraggableExampleApp(),
+    );
+
+    final Finder idleContainer = findContainerWith(
+      color: Colors.lightGreenAccent,
+      child: find.text('Draggable'),
+    );
+    final Finder draggingContainer = findContainerWith(
+      color: Colors.pinkAccent,
+      child: find.text('Child When Dragging'),
+    );
+    final Finder feedbackContainer = findContainerWith(
+      color: Colors.deepOrange,
+      child: find.byIcon(Icons.directions_run),
+    );
+
+    expect(idleContainer, findsOneWidget);
+    expect(draggingContainer, findsNothing);
+    expect(feedbackContainer, findsNothing);
+
+    final TestGesture gesture = await tester.startGesture(
+      tester.getCenter(idleContainer),
+    );
+    await tester.pump();
+
+    expect(idleContainer, findsNothing);
+    expect(draggingContainer, findsOneWidget);
+    expect(feedbackContainer, findsOneWidget);
+
+    await gesture.moveBy(const Offset(200, 0));
+    await tester.pump();
+
+    expect(idleContainer, findsNothing);
+    expect(draggingContainer, findsOneWidget);
+    expect(feedbackContainer, findsOneWidget);
+
+    await gesture.up();
+    await tester.pump();
+
+    expect(idleContainer, findsOneWidget);
+    expect(draggingContainer, findsNothing);
+    expect(feedbackContainer, findsNothing);
+  });
+
+  testWidgets('Dropping Draggable over DragTarget updates the counter', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.DraggableExampleApp(),
+    );
+
+    final Finder draggable = find.byType(Draggable<int>);
+    final Finder target = find.byType(DragTarget<int>);
+
+    int counter = 0;
+
+    for (int i = 0; i < 5; i++) {
+      final TestGesture gesture = await tester.startGesture(
+        tester.getCenter(draggable),
+      );
+      await gesture.moveTo(tester.getCenter(target));
+      await gesture.up();
+      await tester.pump();
+
+      counter += 10;
+
+      expect(
+        findContainerWith(
+          color: Colors.cyan,
+          child: find.text('Value is updated to: $counter'),
+        ),
+        findsOneWidget,
+      );
+    }
+  });
+}