Consolidate encoded transform mocks into api/test/

Includes removing the duplicate MockTransformableAudioFrame definition
in test/ in favour of the existing one in api/test/

Bug: webrtc:15802
Change-Id: Ib5f86b8b2095dd4e580cd9ff0038134f8a43cd93
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/336340
Auto-Submit: Tony Herre <herre@google.com>
Commit-Queue: Stefan Holmer <stefan@webrtc.org>
Reviewed-by: Stefan Holmer <stefan@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#41622}
diff --git a/api/BUILD.gn b/api/BUILD.gn
index 08e33ca..dce687e 100644
--- a/api/BUILD.gn
+++ b/api/BUILD.gn
@@ -1151,6 +1151,16 @@
     ]
   }
 
+  rtc_library("mock_frame_transformer") {
+    visibility = [ "*" ]
+    testonly = true
+    sources = [ "test/mock_frame_transformer.h" ]
+    deps = [
+      ":frame_transformer_interface",
+      "../test:test_support",
+    ]
+  }
+
   rtc_library("mock_encoder_selector") {
     visibility = [ "*" ]
     testonly = true