gbproxy: Introduce TC_llc_discarded

This tests the LLC-DISCARDED message, which relates to a BVCI but
is itself sent on BVCI=0.  We expect it transparently passes from BSS
to SGSN.

Change-Id: I98d02d6fa68bddf15b732d06dab00e91e72995d1
diff --git a/library/Osmocom_Gb_Types.ttcn b/library/Osmocom_Gb_Types.ttcn
index 7894cf1..a345d7f 100644
--- a/library/Osmocom_Gb_Types.ttcn
+++ b/library/Osmocom_Gb_Types.ttcn
@@ -829,7 +829,7 @@
 				iEI := '0F'O,
 				ext := '1'B,
 				lengthIndicator := {
-					length1 := 2
+					length1 := 1
 				},
 				number_of_frames_discarded := f_hex_or_wc(frames_discarded, 2)
 			}