Decouple //rtc_base:rtc_base_tests_utils from gunit.

This CL decouples //rtc_base:rtc_base_tests_utils from gunit by
moving gunit helpers (rtc_base/gunit.h) and rtc_base/testclient.h
(which depends on gunit helpers) to their own build target.

It also removes some unused dependencies in the WebRTC build graph.

Bug: None
Change-Id: Ia9820e84ff697da39b351eef73c45f6e4bdf2623
Reviewed-on: https://webrtc-review.googlesource.com/c/111861
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25769}
diff --git a/sdk/BUILD.gn b/sdk/BUILD.gn
index 4f67d18..60ea473 100644
--- a/sdk/BUILD.gn
+++ b/sdk/BUILD.gn
@@ -1073,8 +1073,8 @@
             "../media:rtc_media_base",
             "../media:rtc_media_tests_utils",
             "../modules:module_api",
+            "../rtc_base:gunit_helpers",
             "../rtc_base:rtc_base",
-            "../rtc_base:rtc_base_tests_utils",
             "../system_wrappers:system_wrappers",
             "//third_party/libyuv",
           ]
@@ -1193,7 +1193,7 @@
           "../modules/audio_processing:api",
           "../modules/audio_processing:audio_processing",
           "../modules/video_coding:video_codec_interface",
-          "../rtc_base:rtc_base_tests_utils",
+          "../rtc_base:gunit_helpers",
           "../rtc_base/system:unused",
           "../test:test_support",
           "//third_party/ocmock",