[a11y-app] Add label to TextFormField in AutoCompleteUseCase. (flutter#175576)

Fixes flutter#173050

### Description

- Adds `labelText` to `TextFormField` in `AutoCompleteUseCase`

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Author: ksokolovskyi

dev/a11y_assessments/lib/use_cases/auto_complete.dart

@@ -36,6 +36,7 @@ class _MainWidgetState extends State<_MainWidget> {
     VoidCallback onFieldSubmitted,
   ) {
     return TextFormField(
+      decoration: const InputDecoration(labelText: 'Fruit'),
       focusNode: focusNode,
       controller: textEditingController,
       onFieldSubmitted: (String value) {

dev/a11y_assessments/test/auto_complete_test.dart

@@ -9,6 +9,11 @@ import 'package:flutter_test/flutter_test.dart';
 import 'test_utils.dart';
 
 void main() {
+  testWidgets('text field label is visible', (WidgetTester tester) async {
+    await pumpsUseCase(tester, AutoCompleteUseCase());
+    expect(find.text('Fruit'), findsOneWidget);
+  });
+
   testWidgets('auto complete can run', (WidgetTester tester) async {
     await pumpsUseCase(tester, AutoCompleteUseCase());
     await tester.enterText(find.byType(TextFormField), 'a');