fix LLC-DISCARD behaviour
Write a ttcn3 testcase which forces the PCU into a LLC discard event.
When the PCU has problems sending data the MS, it might happen the PCU discards LLC PDUs.
The PCU will send a BSSGP LLC DISCARD to the SGSN.
After this the MS become reachable, now it seems the PCU is sending random data with miserable scheduling to the MS.