Re-implementing AcmOpusTest as AcmGenericCodecOpusTest

The old AcmOpusTest depends on the ACMOpus class, but this class was
obsoleted by AudioEncoderOpus. In this CL, the test code is re-written
to use AudioEncoderOpus and ACMGenericCodecWrapper instead of
ACMOpus. Most of the test functionality is preserved, except for the
packet loss rate tests, which where already transferred to
AudioEncoderOpusTest in r8244.

R=kwiberg@webrtc.org, minyue@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/40029004

Cr-Commit-Position: refs/heads/master@{#8410}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8410 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/audio_coding/codecs/opus/interface/audio_encoder_opus.h

@@ -51,6 +51,7 @@ class AudioEncoderOpus final : public AudioEncoder {
   void SetTargetBitrate(int bits_per_second) override;
   void SetProjectedPacketLossRate(double fraction) override;
   double packet_loss_rate() const { return packet_loss_rate_; }
+  ApplicationMode application() const { return application_; }
 
  protected:
   virtual bool EncodeInternal(uint32_t rtp_timestamp,