6.1. Extension for testing hold music

We add the following extension to extensions.conf for testing to see that the music-on-hold is functioning as we expect:
exten => 222,1,Answer()                  ; answer
exten => 222,n,Set(MUSICCLASS()=default) ; set music-on-hold class to "default"
exten => 222,n,WaitMusicOnHold(20)       ; play music-on-hold for 20 seconds
exten => 222,n,Hangup()                  ; hang-up