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/stats/BUILD.gn b/stats/BUILD.gn
index bd19952..c67a5be 100644
--- a/stats/BUILD.gn
+++ b/stats/BUILD.gn
@@ -57,9 +57,9 @@
       ":rtc_stats_test_utils",
       "../api:rtc_stats_api",
       "../rtc_base:checks",
+      "../rtc_base:gunit_helpers",
       "../rtc_base:rtc_base_approved",
       "../rtc_base:rtc_base_tests_main",
-      "../rtc_base:rtc_base_tests_utils",
       "../rtc_base:rtc_json",
     ]