Split the //mojo/public/cpp/test_support:test_utils target.

mojo/public/cpp/system/tests/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_sdk.gni")
 
 mojo_sdk_source_set("tests") {
   testonly = true
 
   sources = [
     "core_unittest.cc",
     "macros_unittest.cc",
   ]
 
   deps = [
     "//testing/gtest",
   ]
 
   mojo_sdk_deps = [
     "mojo/public/c/system/tests",
     "mojo/public/cpp/system",
-    "mojo/public/cpp/test_support:test_utils",
   ]
 }