Fix flaky sliver tree test (flutter#150707)

The set of nodes being used in the tree test was not being consistently
reset at the beginning of every test. The tree is currently broken
because today's random seed for test ordering exposed this leak of state
from one test to another.

Fixes flutter#150706

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Fixes] where supported.
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Author: Piinks

packages/flutter/test/widgets/sliver_tree_test.dart

@@ -28,6 +28,29 @@ List<TreeSliverNode<String>> simpleNodeSet = <TreeSliverNode<String>>[
 
 void main() {
   group('TreeSliverNode', () {
+    setUp(() {
+      // Reset node conditions for each test.
+      simpleNodeSet = <TreeSliverNode<String>>[
+        TreeSliverNode<String>('Root 0'),
+        TreeSliverNode<String>(
+          'Root 1',
+          expanded: true,
+          children: <TreeSliverNode<String>>[
+            TreeSliverNode<String>('Child 1:0'),
+            TreeSliverNode<String>('Child 1:1'),
+          ],
+        ),
+        TreeSliverNode<String>(
+          'Root 2',
+          children: <TreeSliverNode<String>>[
+            TreeSliverNode<String>('Child 2:0'),
+            TreeSliverNode<String>('Child 2:1'),
+          ],
+        ),
+        TreeSliverNode<String>('Root 3'),
+      ];
+    });
+
     test('getters, toString', () {
       final List<TreeSliverNode<String>> children = <TreeSliverNode<String>>[
         TreeSliverNode<String>('child'),
@@ -123,6 +146,7 @@ void main() {
         TreeSliverNode<String>('Root 3'),
       ];
     });
+
     testWidgets('Can set controller on TreeSliver', (WidgetTester tester) async {
       final TreeSliverController controller = TreeSliverController();
       TreeSliverController? returnedController;
@@ -427,6 +451,26 @@ void main() {
   });
 
   testWidgets('.toggleNodeWith, onNodeToggle', (WidgetTester tester) async {
+    simpleNodeSet = <TreeSliverNode<String>>[
+      TreeSliverNode<String>('Root 0'),
+      TreeSliverNode<String>(
+        'Root 1',
+        expanded: true,
+        children: <TreeSliverNode<String>>[
+          TreeSliverNode<String>('Child 1:0'),
+          TreeSliverNode<String>('Child 1:1'),
+        ],
+      ),
+      TreeSliverNode<String>(
+        'Root 2',
+        children: <TreeSliverNode<String>>[
+          TreeSliverNode<String>('Child 2:0'),
+          TreeSliverNode<String>('Child 2:1'),
+        ],
+      ),
+      TreeSliverNode<String>('Root 3'),
+    ];
+
     final TreeSliverController controller = TreeSliverController();
     // The default node builder wraps the leading icon with toggleNodeWith.
     bool toggled = false;
@@ -516,6 +560,26 @@ void main() {
   });
 
   testWidgets('AnimationStyle is piped through to node builder', (WidgetTester tester) async {
+    simpleNodeSet = <TreeSliverNode<String>>[
+      TreeSliverNode<String>('Root 0'),
+      TreeSliverNode<String>(
+        'Root 1',
+        expanded: true,
+        children: <TreeSliverNode<String>>[
+          TreeSliverNode<String>('Child 1:0'),
+          TreeSliverNode<String>('Child 1:1'),
+        ],
+      ),
+      TreeSliverNode<String>(
+        'Root 2',
+        children: <TreeSliverNode<String>>[
+          TreeSliverNode<String>('Child 2:0'),
+          TreeSliverNode<String>('Child 2:1'),
+        ],
+      ),
+      TreeSliverNode<String>('Root 3'),
+    ];
+
     AnimationStyle? style;
     await tester.pumpWidget(MaterialApp(
       home: CustomScrollView(