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/BUILD.gn b/modules/video_coding/BUILD.gn
index b654128..afa677d 100644
--- a/modules/video_coding/BUILD.gn
+++ b/modules/video_coding/BUILD.gn
@@ -605,6 +605,8 @@
       ":video_coding",
       ":video_coding_utility",
       "../../:webrtc_common",
+      "../../api:mock_video_decoder",
+      "../../api:mock_video_encoder",
       "../../api:simulcast_test_fixture_api",
       "../../api/video:encoded_image",
       "../../api/video:video_frame",
@@ -763,6 +765,8 @@
       "../..:webrtc_common",
       "../../api:create_videocodec_test_fixture_api",
       "../../api:mock_video_codec_factory",
+      "../../api:mock_video_decoder",
+      "../../api:mock_video_encoder",
       "../../api:videocodec_test_fixture_api",
       "../../api/video:video_frame",
       "../../api/video:video_frame_i420",
@@ -882,6 +886,8 @@
       "..:module_api",
       "../..:webrtc_common",
       "../../api:create_simulcast_test_fixture_api",
+      "../../api:mock_video_decoder",
+      "../../api:mock_video_encoder",
       "../../api:simulcast_test_fixture_api",
       "../../api:videocodec_test_fixture_api",
       "../../api/video:video_bitrate_allocator",