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.cc

@@ -77,6 +77,8 @@ absl::optional<SdpAudioFormat> NetEqDecoderToSdpAudioFormat(NetEqDecoder nd) {
       return SdpAudioFormat("cn", 32000, 1);
     case NetEqDecoder::kDecoderCNGswb48kHz:
       return SdpAudioFormat("cn", 48000, 1);
+    case NetEqDecoder::kDecoderReplacementForTest:
+      return SdpAudioFormat("replacement", 48000, 1);
     default:
       return absl::nullopt;
   }