pjsip/rest_api: Introduce a test for ChannelTransfer event

[zecke]

Test that a SIP REFER is translated into a ChannelTransfer event when TRANSFERHANDLING=ari-only is set on the channel and that transfer_progress generates the necessary SIP NOTIFY messages.

Be strict on the first ChannelTransfer event being received. Then only check that the transfer state is changing from not being present, to progress and finally answered. When answered delete all the channels and the bridge and the test wull succeed.