msc: add TC_mo_ussd_evil_ms for OS#6756This is an "evil MS" scenario that crashes current osmo-msc.Change-Id: If1d85a1b4b63b01b4565e53677acfd21e664e799Related: osmo-msc.git I724f0f0c9ef8611d3c3653e9370361b252127f72Related: OS#6756