kmsaudiomixer: Add a audiotestsrc always connected to the audiomixer
kmsaudiomixer: Add a audiotestsrc always connected to the audiomixer

Change-Id: Iec8c5305476967443b006ff32b66dadaa0bb9553

shelsonjava/kms-core,Kurento/kms-core,Kurento/kms-core,ESTOS/kms-core,TribeMedia/kms-core,ESTOS/kms-core,shelsonjava/kms-core,shelsonjava/kms-core,TribeMedia/kms-core,Kurento/kms-core,TribeMedia/kms-core,ESTOS/kms-core,TribeMedia/kms-core,Kurento/kms-core,ESTOS/kms-core,shelsonjava/kms-core