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_sender_unittest.cc b/modules/video_coding/video_sender_unittest.cc
index eb3c09c..fb55dc9 100644
--- a/modules/video_coding/video_sender_unittest.cc
+++ b/modules/video_coding/video_sender_unittest.cc
@@ -11,11 +11,11 @@
 #include <memory>
 #include <vector>
 
+#include "api/test/mock_video_encoder.h"
 #include "api/video/i420_buffer.h"
 #include "api/video_codecs/vp8_temporal_layers.h"
 #include "modules/video_coding/codecs/vp8/include/vp8.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/utility/default_video_bitrate_allocator.h"
 #include "modules/video_coding/utility/simulcast_rate_allocator.h"