EDK: Split the mojo_edk_system_unittests target.

mojo/edk/embedder/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("embedder") {
   # This isn't really a standalone target; it must be linked into the
   # mojo_system_impl component.
   mojo_edk_visibility = [ "mojo/edk/system" ]
@@ skipping 90 matching lines @@
     "process_type.h",
     "slave_info.h",
     "slave_process_delegate.h",
   ]
 
   mojo_edk_configs = [ "mojo/edk/system:system_config" ]
 
   mojo_sdk_public_deps = [ "mojo/public/cpp/system" ]
 }
 
-mojo_edk_source_set("unittests") {
-  testonly = true
-  mojo_edk_visibility = [ "mojo/edk/system:mojo_edk_system_unittests" ]
-
+mojo_edk_unittests("mojo_edk_embedder_unittests") {
   sources = [
     "embedder_unittest.cc",
     "platform_channel_pair_unittest.cc",
     "simple_platform_shared_buffer_unittest.cc",
   ]
 
   deps = [
     "//base",
     "//testing/gtest",
   ]
 
   mojo_edk_deps = [
     "mojo/edk/test:test_support",
     "mojo/edk/system",
     "mojo/edk/system/test",
     "mojo/edk/util",
   ]
 }