Add lots of testonly annotations to the GN build.

rlz/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.
 
 # Note that this build file assumes rlz_use_chrome_net which is a condition in
 # the GYP file, but is always true for Chrome builds.
 
 config("rlz_config") {
   defines = [ "RLZ_NETWORK_IMPLEMENTATION_CHROME_NET" ]
 }
@@ skipping 58 matching lines @@
       "mac/lib/rlz_value_store_mac.h",
     ]
     set_sources_assignment_filter(sources_assignment_filter)
   } else if (is_win) {
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     cflags = [ "/wd4267" ]
   }
 }
 
 source_set("test_support") {
+  testonly = true
   sources = [
     "test/rlz_test_helpers.cc",
     "test/rlz_test_helpers.h",
   ]
   deps = [
     ":rlz_lib",
     "//base",
     "//base/test:test_support",
     "//testing/gtest",
   ]
@@ skipping 38 matching lines @@
     sources = [
       "win/dll/dll_main.cc",
       "win/dll/exports.cc",
     ]
     deps = [
       ":rlz_lib",
       "//third_party/zlib",
     ]
   }
 }