Add lots of testonly annotations to the GN build.

components/proximity_auth/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.
 
 static_library("proximity_auth") {
   sources = [
     "proximity_auth_system.cc",
     "proximity_auth_system.h",
     "remote_device.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "proximity_auth_system_unittest.cc",
   ]
 
   deps = [
     ":proximity_auth",
     "//base/test:test_support",
     "//testing/gtest",
   ]
 }