MSC: match default expectation with config

In MSC_Tests.default we expect /tmp/mncc.sock as MNCC socket path -
let's match this expectation with osmo-msc.cfg to make sure that tests
work out of the box without the need to use specific command-line
option.

Change-Id: I540645ef4b1e08d05b89251f074af84a516e7a88

msc/osmo-msc.cfg

@@ -73,6 +73,7 @@ msc
  cs7-instance-iu 0
  mgw remote-ip 127.0.0.1
  emergency-call route-to-msisdn 112
+ mncc external /tmp/mncc.sock
 mncc-int
  default-codec tch-f fr
  default-codec tch-h hr