Move MockVideoDecoder to api/test.

Move MockVideoDecoder from
modules/video_coding/include/mock/mock_video_codec_interface.h
to
api/test/mock_video_decoder.h

The mock encoder has already moved:
https://webrtc-review.googlesource.com/c/src/+/105620

Keeping the old header until downstream projects have been updated.

Bug: webrtc:9722
Change-Id: I4bc849173a04813064212f17761876695ca3fed4
Reviewed-on: https://webrtc-review.googlesource.com/c/105900
Reviewed-by: Niels Moller <nisse@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25170}
diff --git a/modules/video_coding/video_receiver_unittest.cc b/modules/video_coding/video_receiver_unittest.cc
index b7a46f9..f50398c 100644
--- a/modules/video_coding/video_receiver_unittest.cc
+++ b/modules/video_coding/video_receiver_unittest.cc
@@ -11,8 +11,8 @@
 #include <memory>
 #include <vector>
 
+#include "api/test/mock_video_decoder.h"
 #include "modules/video_coding/include/mock/mock_vcm_callbacks.h"
-#include "modules/video_coding/include/mock/mock_video_codec_interface.h"
 #include "modules/video_coding/include/video_coding.h"
 #include "modules/video_coding/test/test_util.h"
 #include "modules/video_coding/timing.h"