Fix leaky tests. (flutter#150817)

Author: polina-c

packages/flutter/test/cupertino/button_test.dart

@@ -492,6 +492,8 @@ void main() {
 
   testWidgets('Button can be focused and has default colors', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode(debugLabel: 'Button');
+    addTearDown(focusNode.dispose);
+
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
     const Border defaultFocusBorder = Border.fromBorderSide(
       BorderSide(
@@ -540,6 +542,8 @@ void main() {
 
   testWidgets('Button configures focus color', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode(debugLabel: 'Button');
+    addTearDown(focusNode.dispose);
+
     tester.binding.focusManager.highlightStrategy = FocusHighlightStrategy.alwaysTraditional;
     const Color focusColor = CupertinoColors.systemGreen;
 
@@ -576,6 +580,8 @@ void main() {
 
   testWidgets('CupertinoButton.onFocusChange callback', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode(debugLabel: 'CupertinoButton');
+    addTearDown(focusNode.dispose);
+
     bool focused = false;
     await tester.pumpWidget(
       CupertinoApp(

packages/flutter/test/widgets/undo_history_test.dart

@@ -32,6 +32,7 @@ void main() {
 
     testWidgets('UndoHistory widget registers as global undo/redo client', (WidgetTester tester) async {
       final FocusNode focusNode = FocusNode(debugLabel: 'UndoHistory Node');
+      addTearDown(focusNode.dispose);
       final GlobalKey undoHistoryGlobalKey = GlobalKey();
       final ValueNotifier<int> value = ValueNotifier<int>(0);
       addTearDown(value.dispose);
@@ -68,6 +69,7 @@ void main() {
     testWidgets('UndoHistory widget deregisters as global undo/redo client when it loses focus',
             (WidgetTester tester) async {
           final FocusNode focusNode = FocusNode(debugLabel: 'UndoHistory Node');
+          addTearDown(focusNode.dispose);
           final GlobalKey undoHistoryGlobalKey = GlobalKey();
           final ValueNotifier<int> value = ValueNotifier<int>(0);
           addTearDown(value.dispose);
@@ -105,6 +107,7 @@ void main() {
 
     testWidgets('UndoHistory widget deregisters as global undo/redo client when disposed', (WidgetTester tester) async {
       final FocusNode focusNode = FocusNode(debugLabel: 'UndoHistory Node');
+      addTearDown(focusNode.dispose);
       final GlobalKey undoHistoryGlobalKey = GlobalKey();
       final ValueNotifier<int> value = ValueNotifier<int>(0);
       addTearDown(value.dispose);