Add a unit test for NavigationRail (#143108)

Adding `SingleChildScrollView` to `NavigationRail` may cause exception if the nav rail has some expanded widgets inside, like the issue: https://github.com/flutter/flutter/issues/143061. This PR is just to add a unit test to avoid causing this breaking again.

packages/flutter/test/material/navigation_rail_test.dart

@@ -3605,6 +3605,32 @@ void main() {
     expect(inkFeatures, paints..circle(color: Colors.transparent));
   }, skip: kIsWeb && !isCanvasKit); // https://github.com/flutter/flutter/issues/99933
 
+  testWidgets('Navigation rail can have expanded widgets inside', (WidgetTester tester) async {
+    await _pumpNavigationRail(
+      tester,
+      navigationRail: NavigationRail(
+        selectedIndex: 0,
+        destinations: const <NavigationRailDestination>[
+          NavigationRailDestination(
+            icon: Icon(Icons.favorite_border),
+            label: Text('Abc'),
+          ),
+          NavigationRailDestination(
+            icon: Icon(Icons.bookmark_border),
+            label: Text('Bcd'),
+          ),
+        ],
+        trailing: const Expanded(
+          child: Icon(Icons.search),
+        ),
+      ),
+    );
+
+    await tester.pumpAndSettle();
+
+    expect(tester.takeException(), isNull);
+  });
+
   group('Material 2', () {
     // These tests are only relevant for Material 2. Once Material 2
     // support is deprecated and the APIs are removed, these tests