Add lots of testonly annotations to the GN build.

mojo/common/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.
 
 # GYP version: mojo/mojo_base.gyp:mojo_run_all_unittests
 source_set("run_all_unittests") {
+  testonly = true
   deps = [
     ":test_support_impl",
     "//base",
     "//base/test:test_support",
     "//mojo/public/c/test_support",
     "//mojo/system",
     "//testing/gtest",
   ]
 
   sources = [ "run_all_unittests.cc" ]
 }
 
 # GYP version: mojo/mojo_base.gyp:mojo_test_support_impl
 source_set("test_support_impl") {
+  testonly = true
   deps = [
     "//base",
   ]
 
   sources = [
     "test_support_impl.cc",
     "test_support_impl.h",
   ]
 }
 
 # GYP version: mojo/mojo_base.gyp:mojo_common_test_support
 source_set("test_support") {
+  testonly = true
   deps = [
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
     "//mojo/system",
   ]
 
   sources = [
     "multiprocess_test_helper.cc",
     "multiprocess_test_helper.h",
     "test_utils.h",
     "test_utils_posix.cc",
     "test_utils_win.cc",
   ]
 }