Changes

SGsAP_Emulation: Introduce proper support for server-mode

Prior to this commit, only initial (not really useful) SCTP server-mode
support existed in SGsAP_Emulation.
This is basically because MSC_Tests uses the SCTP client-mode, and
MME_Tests_SGsAP were not yet in a fully working state.

In server-mode, we have a conn_id for the listening socket and a conn_id
for the accepted socket; track them properly.
Moreover, in server node we definetly need some sort of event to wait
for the client to connect; introduce it.
Based on existing work in Iuh_Emulation.

Change-Id: Iccf4ac96c56e947529f0ffc06428e2325a115d50

mme: Initial working state for MME_Tests_SGsAP.ttcn tests

MME_Tests_SGsAP.ttcn tests were introduced to the repository many years
ago (~2019), but they were mostly WIP and were never run as part of the
regular testsuite.

This commit reworks and extends previous work to get them up to date
with current MME_Tests and makes most of them passing.

TC_sgsap_alert_* tests are still in WIP and will be fixed in follow-up
work.

Related: OS#6850
Change-Id: If07f1a67eb4213985da82afec94adb08786a0980