Convert some GN test support targets to static libs.

mojo/public/cpp/test_support/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.
 
 # GYP version: mojo/public/mojo_public.gyp:mojo_public_test_utils
-source_set("test_utils") {
+static_library("test_utils") {
   testonly = true
 
   sources = [
     "lib/test_support.cc",
     "lib/test_utils.cc",
     "test_utils.h",
   ]
 
   deps = [
     "//mojo/public/c/test_support",
     "//mojo/public/cpp/system",
     "//testing/gtest",
   ]
 }