MME_Tests: add Gn interface

To test the planned Gn connectivity support in open5gs MME the testsuite
also requires support for such an interface. This patch adds the
connection handler, provided by GTP_Emulation.ttcn to the connection
handler in MME_Tests.ttcn, along with a simple GTP ECHO REQUEST
testcase.

Related: OS#5760
Change-Id: I38b668df15b3dd10542b4aa8790b9ea33c1f9635
3 files changed