Reenable some sample tests and fix how they find their data files

samples/sample_extension/test/sample_extension_app_snapshot_test.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Dart test program for testing native extensions.
 
+// OtherResources=../sample_synchronous_extension.dart
+// OtherResources=../sample_asynchronous_extension.dart
+// OtherResources=../test_sample_synchronous_extension.dart
+// OtherResources=../test_sample_asynchronous_extension.dart
+
 import 'sample_extension_test_helper.dart';
 
 void main() {
   testNativeExtensions("app-jit");
 }