Fully qualify googletest symbols.

Semi-automatically created with:

git grep -l " testing::" | xargs sed -i "s/ testing::/ ::testing::/g"
git grep -l "(testing::" | xargs sed -i "s/(testing::/(::testing::/g"
git cl format

After this, two .cc files failed to compile and I have fixed them
manually.

Bug: webrtc:10523
Change-Id: I4741d3bcedc831b6c5fdc04485678617eb4ce031
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/132018
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27526}
diff --git a/modules/bitrate_controller/bitrate_controller_unittest.cc b/modules/bitrate_controller/bitrate_controller_unittest.cc
index cdb9f30..0f86210 100644
--- a/modules/bitrate_controller/bitrate_controller_unittest.cc
+++ b/modules/bitrate_controller/bitrate_controller_unittest.cc
@@ -94,7 +94,7 @@
   TestBitrateObserver bitrate_observer_;
   std::unique_ptr<BitrateController> controller_;
   RtcpBandwidthObserver* bandwidth_observer_;
-  testing::NiceMock<webrtc::MockRtcEventLog> event_log_;
+  ::testing::NiceMock<webrtc::MockRtcEventLog> event_log_;
 };
 
 TEST_F(BitrateControllerTest, DefaultMinMaxBitrate) {