Fix test Renegotiation_AddTrack_Multiple
Fix test Renegotiation_AddTrack_Multiple

Fixes #1176. I was unable to reproduce this locally, but this must've
happened because onTrackChan was being written to before onTrackCount
variable was updated. I changed the order and removed the unnecessary
lock.

pion/webrtc,pion/webrtc,pion/webrtc,pion/webrtc