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

mojo/edk/system/test/BUILD.gn

 # Copyright 2015 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") {
   testonly = true
 
   sources = [
     "random.cc",
     "random.h",
+    "scoped_test_dir.cc",
+    "scoped_test_dir.h",
     "simple_test_thread.cc",
     "simple_test_thread.h",
     "sleep.cc",
     "sleep.h",
     "stopwatch.cc",
     "stopwatch.h",
+    "test_io_thread.cc",
+    "test_io_thread.h",
     "timeouts.cc",
     "timeouts.h",
   ]
 
   mojo_sdk_public_deps = [
     "mojo/public/c/system",
     "mojo/public/cpp/system",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
   ]
 
-  mojo_edk_deps = [ "mojo/edk/embedder:platform" ]
+  mojo_edk_deps = [
+    "mojo/edk/embedder:platform",
+    "mojo/edk/util",
+  ]
 }
 
 mojo_edk_source_set("unittests") {
   testonly = true
   mojo_edk_visibility = [ "mojo/edk/system:mojo_system_unittests" ]
 
   sources = [
     "random_unittest.cc",
   ]
 
   deps = [
     ":test",
     "//testing/gtest",
   ]
 }