GN: Tighten up test target visibility + refactorings

webrtc/test/BUILD.gn

 # Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 #
 # Use of this source code is governed by a BSD-style license
 # that can be found in the LICENSE file in the root of the source
 # tree. An additional intellectual property rights grant can be found
 # in the file PATENTS.  All contributing project authors may
 # be found in the AUTHORS file in the root of the source tree.
 
 import("../webrtc.gni")
 import("//build/config/ui.gni")
@@ skipping 290 matching lines @@
     ":video_test_common",
     ":video_test_support",
     "../modules/video_capture",
     "//testing/gmock",
     "//testing/gtest",
     "//third_party/gflags",
   ]
 }
 rtc_source_set("fileutils_unittests") {
   testonly = true
+  visibility = [ ":*" ]  # Only targets in this file can depend on this.
   sources = [
     "testsupport/fileutils_unittest.cc",
   ]
   deps = [
     ":fileutils",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
@@ skipping 168 matching lines @@
   ]
 }
 
 rtc_source_set("audio_codec_mocks") {
   testonly = true
   sources = [
     "mock_audio_decoder.h",
     "mock_audio_decoder_factory.h",
   ]
 }