Add a mojo::test::ScopedTestDir and convert tests to use it.

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")
 import("../../public/mojo.gni")
 import("//testing/test.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.h",
     "test_utils_posix.cc",
     "test_utils_win.cc",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
@@ skipping 132 matching lines @@
     "../../public/cpp/utility/tests",
   ]
 }
 
 test("mojo_system_impl_private_unittests") {
   deps = [
     ":run_all_unittests",
     "../../public/platform/native:system_impl_private_tests",
   ]
 }