Changes

mgcp_test.c: verify osmo-mgw accepts m=audio 0

When osmo-hnbgw does not yet know the remote port, it wants to send a
CRCX to set up IuUP at the MGW, with the audio port set to zero.
Make sure osmo-mgw accepts port == 0.

Related: SYS#6907 SYS#6974
Change-Id: I42011c2e7256d372a37b6a2fe86af0153038e2d0