Move scoped_test_dir.* and test_io_thread.* from edk/test to edk/system/test.

mojo/edk/test/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../mojo_edk.gni")
 
 mojo_edk_source_set("test_support") {
   testonly = true
   sources = [
     "multiprocess_test_helper.cc",
     "multiprocess_test_helper.h",
     "scoped_ipc_support.cc",
     "scoped_ipc_support.h",
-    "scoped_test_dir.cc",
-    "scoped_test_dir.h",
-    "test_io_thread.cc",
-    "test_io_thread.h",
     "test_utils.cc",
     "test_utils.h",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 
@@ skipping 49 matching lines @@
 
   sources = [
     "test_support_impl.cc",
   ]
 
   visibility = [
     ":run_all_unittests",
     ":run_all_perftests",
   ]
 }