Added missing tests for ButtonStyle example (flutter#147457)

Added missing tests for ButtonStyle example. Issue flutter#130459

Author: osama383

dev/bots/check_code_samples.dart

@@ -317,7 +317,6 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/selectable_region/selectable_region.0_test.dart',
   'examples/api/test/material/text_field/text_field.2_test.dart',
   'examples/api/test/material/text_field/text_field.1_test.dart',
-  'examples/api/test/material/button_style/button_style.0_test.dart',
   'examples/api/test/material/range_slider/range_slider.0_test.dart',
   'examples/api/test/material/selection_container/selection_container_disabled.0_test.dart',
   'examples/api/test/material/selection_container/selection_container.0_test.dart',

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

@@ -0,0 +1,74 @@
+// 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/button_style/button_style.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets(
+      'Shows ElevatedButtons, FilledButtons, OutlinedButtons and TextButtons in enabled and disabled states',
+      (WidgetTester tester) async {
+    await tester.pumpWidget(const example.ButtonApp());
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is ElevatedButton && widget.onPressed == null;
+      }),
+      findsOne,
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is ElevatedButton && widget.onPressed != null;
+      }),
+      findsOne,
+    );
+
+    // One OutlinedButton with onPressed null.
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is OutlinedButton && widget.onPressed == null;
+      }),
+      findsOne,
+    );
+
+    // One OutlinedButton with onPressed not null.
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is OutlinedButton && widget.onPressed != null;
+      }),
+      findsOne,
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is TextButton && widget.onPressed == null;
+      }),
+      findsOne,
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is TextButton && widget.onPressed != null;
+      }),
+      findsOne,
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is FilledButton && widget.onPressed != null;
+      }),
+      findsNWidgets(2),
+    );
+
+    expect(
+      find.byWidgetPredicate((Widget widget) {
+        return widget is FilledButton && widget.onPressed == null;
+      }),
+      findsNWidgets(2),
+    );
+  });
+}