Move all tests using dart:io to a separate directory

tests/standalone/src/io/ProcessWorkingDirectoryTest.dart

 // Copyright (c) 2012, 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.
 //
 // Process working directory test.
 
 #library("ProcessWorkingDirectoryTest");
 #import("dart:io");
 #source("ProcessTestUtil.dart");
 
@@ skipping 47 matching lines @@
     };
   }
 }
 
 
 
 main() {
   ProcessWorkingDirectoryTest.testValidDirectory();
   ProcessWorkingDirectoryTest.testInvalidDirectory();
 }