updated tearDownAll function (#28808)

dev/integration_tests/ui/test_driver/commands_debug_paint_test.dart

@@ -13,7 +13,7 @@ void main() {
   });
 
   tearDownAll(() async {
-    driver?.close();
+    await driver?.close();
   });
 
   test('check that we are painting in debugPaintSize mode', () async {

dev/integration_tests/ui/test_driver/commands_none_test.dart

@@ -13,7 +13,7 @@ void main() {
   });
 
   tearDownAll(() async {
-    driver?.close();
+    await driver?.close();
   });
 
   test('check that we are in normal mode', () async {

dev/integration_tests/ui/test_driver/commands_performance_overlay_test.dart

@@ -13,7 +13,7 @@ void main() {
   });
 
   tearDownAll(() async {
-    driver?.close();
+    await driver?.close();
   });
 
   test('check that we are showing the performance overlay', () async {

dev/integration_tests/ui/test_driver/keyboard_resize_test.dart

@@ -18,7 +18,7 @@ void main() {
     });
 
     tearDownAll(() async {
-      driver?.close();
+      await driver?.close();
     });
 
     test('Ensure keyboard dismissal resizes the view to original size', () async {

dev/integration_tests/ui/test_driver/keyboard_textfield_test.dart

@@ -18,7 +18,7 @@ void main() {
     });
 
     tearDownAll(() async {
-      driver?.close();
+      await driver?.close();
     });
 
     test('Textfield scrolls back into view after covered by keyboard', () async {

dev/integration_tests/ui/test_driver/route_test.dart

@@ -15,7 +15,7 @@ void main() {
     });
 
     tearDownAll(() async {
-      driver?.close();
+      await driver?.close();
     });
 
     test('sanity check flutter drive --route', () async {