Refactor NetEqTestFactory to not use "external" decoders

Bug: webrtc:10080
Change-Id: Icfca98d6d91fc5139e678c1aa3de1e2c35abff5c
Reviewed-on: https://webrtc-review.googlesource.com/c/115240
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26090}

modules/audio_coding/neteq/neteq_decoder_enum.h

@@ -16,6 +16,7 @@
 
 namespace webrtc {
 
+// TODO(bugs.webrtc.org/10080): Delete, together with RegisterExternalDecoder.
 enum class NetEqDecoder {
   kDecoderPCMu,
   kDecoderPCMa,
@@ -47,6 +48,8 @@ enum class NetEqDecoder {
   kDecoderArbitrary,
   kDecoderOpus,
   kDecoderOpus_2ch,
+  // Used in NetEqTestFactory
+  kDecoderReplacementForTest,
 };
 
 absl::optional<SdpAudioFormat> NetEqDecoderToSdpAudioFormat(NetEqDecoder nd);