Renamed appbar to app_bar directory in API Examples Tests (#139922)

Tests for `app_bar.0`, `app_bar.1`, `app_bar.2`, `app_bar.3`, `sliver_app_bar.1` and `sliver_app_bar.4` were already present. But directory name was `appbar` rather than `app_bar`. I've renamed the directory to `app_bar` since example files uses that only.

Part of #130459

dev/bots/check_code_samples.dart

@@ -367,14 +367,8 @@ final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/scaffold/scaffold_messenger.of.1_test.dart',
   'examples/api/test/material/scaffold/scaffold_messenger_state.show_snack_bar.0_test.dart',
   'examples/api/test/material/segmented_button/segmented_button.0_test.dart',
-  'examples/api/test/material/app_bar/app_bar.2_test.dart',
-  'examples/api/test/material/app_bar/sliver_app_bar.1_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.2_test.dart',
   'examples/api/test/material/app_bar/sliver_app_bar.3_test.dart',
-  'examples/api/test/material/app_bar/app_bar.1_test.dart',
-  'examples/api/test/material/app_bar/sliver_app_bar.4_test.dart',
-  'examples/api/test/material/app_bar/app_bar.3_test.dart',
-  'examples/api/test/material/app_bar/app_bar.0_test.dart',
   'examples/api/test/material/banner/material_banner.1_test.dart',
   'examples/api/test/material/banner/material_banner.0_test.dart',
   'examples/api/test/material/checkbox/checkbox.1_test.dart',