Clean leak in editable_text_test.dart. (#149223)

Introduced by flutter/flutter#149042

<img width="145" alt="Screenshot 2024-05-28 at 6 25 22�PM" src="https://github.com/flutter/flutter/assets/12115586/5eb32012-b1c4-4188-8e9b-a6631483960c">

Author: polina-c

packages/flutter/test/widgets/editable_text_test.dart

@@ -17656,6 +17656,8 @@ void main() {
   });
 
   testWidgets('Can implement TextEditingController', (WidgetTester tester) async {
+    final _TextEditingControllerImpl controller = _TextEditingControllerImpl();
+    addTearDown(controller.dispose);
     await tester.pumpWidget(
       MediaQuery(
         data: const MediaQueryData(),
@@ -17664,7 +17666,7 @@ void main() {
           child: EditableText(
             autofocus: true,
             backgroundCursorColor: Colors.grey,
-            controller: _TextEditingControllerImpl(),
+            controller: controller,
             focusNode: focusNode,
             style: textStyle,
             cursorColor: cursorColor,