Clean leaky test. (#149199)

Introduced by https://github.com/flutter/flutter/pull/149006

To repro run:

`flutter test /Users/polinach/_/flutter_dev/packages/flutter/test/material/text_button_test.dart --dart-define LEAK_TR`

<img width="268" alt="Screenshot 2024-05-28 at 11 50 33 AM" src="https://github.com/flutter/flutter/assets/12115586/c1a1bf5e-4df4-4819-b897-b09021d3a07a">

packages/flutter/test/material/text_button_test.dart

diff --git a/packages/flutter/test/material/text_button_test.dart b/packages/flutter/test/material/text_button_test.dart
index e15a7a7..f213635 100644
--- a/packages/flutter/test/material/text_button_test.dart
+++ b/packages/flutter/test/material/text_button_test.dart
@@ -2303,6 +2303,7 @@
 
   testWidgets('treats a hovering stylus like a mouse', (WidgetTester tester) async {
     final FocusNode focusNode = FocusNode();
+    addTearDown(focusNode.dispose);
     final ThemeData theme = ThemeData(useMaterial3: true);
     bool hasBeenHovered = false;