Changes

msc: f_tc_ho_inter_bsc0(): catch BSSMAP HandoverRequiredReject

It may happen that the target BSC is not yet connected to the MSC.
In this case the MSC rejects handover with HandoverRequiredReject.
Let's catch this PDU to make the verdict cleaner in such case.

Change-Id: I5ba98e323e0c7794554ccb115d7697ad03ccbfa5
Related: OS#6951

msc: TC_ho_inter_bsc: wait for the target BSC to become ready

A race condition may occur between the two testcase components
emulating BSCs: the source BSC initiates handover to the terget
BSC before the later completes connection establishment with the
MSC.  In this case, the MSC fails to find the target peer and
rejects our HandoverRequired with HandoverRequiredReject.

  Target RAN peer from neighbor config is not connected: Cell ID CGI:023-42-5-6 resolves to target address 0.24.2
  Cannot find target peer for cell ID CGI:023-42-5-6
  Attempted Handover to 1 cells without success

Let's avoid this by waiting for the target BSC to become available
before initiating the handover procedure.  Use the COORD port
("intercom") to signal readiness of the target BSC.

Change-Id: I00b8a8b89ec7bb15590d30aadf40055981375a5d
Related: OS#6951