Reset the global variable before running a test. (#46203)

The variable debugDisableShadows wasn't reset before each test, so the
output of the test is different depending on whether the previous test
has passed.
diff --git a/packages/flutter/test/widgets/shadow_test.dart b/packages/flutter/test/widgets/shadow_test.dart
index e7260e3..f95f753 100644
--- a/packages/flutter/test/widgets/shadow_test.dart
+++ b/packages/flutter/test/widgets/shadow_test.dart
@@ -6,6 +6,10 @@
 import 'package:flutter/material.dart';
 
 void main() {
+  tearDown(() {
+    debugDisableShadows = true;
+  });
+
   testWidgets('Shadows on BoxDecoration', (WidgetTester tester) async {
     await tester.pumpWidget(
       Center(