Added missing test for SelectionArea example (#151585)

Added missing test for SelectionArea example. Issue #130459

examples/api/test/material/selection_area/selection_area.0_test.dart

@@ -0,0 +1,40 @@
+// 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/material/selection_area/selection_area.0.dart' as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('Texts are descendant of the SelectionArea', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.SelectionAreaExampleApp(),
+    );
+
+    expect(
+      find.descendant(
+        of: find.byType(SelectionArea),
+        matching: find.byType(Text),
+      ),
+      findsExactly(4),
+    );
+
+    final List<String> selectableTexts = <String>[
+      'SelectionArea Sample',
+      'Row 1',
+      'Row 2',
+      'Row 3',
+    ];
+
+    for (final String text in selectableTexts) {
+      expect(
+        find.descendant(
+          of: find.byType(SelectionArea),
+          matching: find.text(text),
+        ),
+        findsExactly(1),
+      );
+    }
+  });
+}