Add more tests for .packages

tests/standalone/package/scenarios/invalid/non_existent_packages_file_test.dart

 // Copyright (c) 2015, 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.
 
 // PackageRoot=none
+// Packages=non_existent.packages
 
-library packages_dir_only_noimports_test;
+library non_existent_packages_file_test;
 
 main() {
 }