[GAR] [a11y] Update text button page in a11y assessment app (#148905)

internal: b/317125569

If the text button label contains its role and state, it will be confusing for tester.

it will be announced like "xxx button button" :

<img src="https://github.com/flutter/flutter/assets/108393416/16f5a996-b0f1-4f04-bd99-caa3cacd4182" width=50% height=50%> 

updated screen and label: 
<img src="https://github.com/flutter/flutter/assets/108393416/1de55cec-8a18-4c36-9dac-84ae7d151fd6" width=50% height=50%>

Author: hannah-hyj

dev/a11y_assessments/lib/use_cases/text_button.dart

@@ -39,13 +39,25 @@ class MainWidgetState extends State<MainWidget> {
         child: Column(
           mainAxisAlignment: MainAxisAlignment.center,
           children: <Widget>[
-            TextButton(
-              onPressed: () {  },
-              child: const Text('Text button'),
+            Row(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: <Widget>[
+                const Text('This is a TextButton:'),
+                TextButton(
+                  onPressed: () {  },
+                  child: const Text('Action'),
+                ),
+              ],
             ),
-            const TextButton(
-              onPressed: null,
-              child: Text('Text button disabled'),
+            const Row(
+              mainAxisAlignment: MainAxisAlignment.center,
+              children: <Widget>[
+                Text('This is a disabled TextButton:'),
+                TextButton(
+                  onPressed: null,
+                  child: Text('Action'),
+                ),
+              ],
             ),
           ],
         ),

dev/a11y_assessments/test/text_button_test.dart

@@ -10,7 +10,6 @@ import 'test_utils.dart';
 void main() {
   testWidgets('text button can run', (WidgetTester tester) async {
     await pumpsUseCase(tester, TextButtonUseCase());
-    expect(find.text('Text button'), findsOneWidget);
-    expect(find.text('Text button disabled'), findsOneWidget);
+    expect(find.text('Action'), findsExactly(2));
   });
 }