BSSGP_Emulation: add BssgpDecodeDepth

Make the decoding level (BSSGP, LLC, SNDCP, L3) configurable, so the
existing PCU tests, that expect messages only decoded to the BSSGP
level, can pass again. Move the SNDCP decoding in f_dec_bssgp above the
L3 decoding, so f_dec_bssgp goes through the layers in the reverse order
of f_send_bssgp_dec.

I have verified, that all testsuites using the BSSGP Emulation (SGSN,
PCU, PCU-SNS) are still working with this patch.

Related: OS#4180
Fixes: 955aa94504510139a12d223071cf49ef90788a3d ("BSSGP_Emulation: Abandon "BssgpDecoded" intermediate structure")
Change-Id: I8f76385528c1de98c557cee451c0e0dfd182b605
diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index db7d54b..b68df85 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -286,7 +286,8 @@
 				},
 			cell_id := 20960
 		},
-		sgsn_role := false
+		sgsn_role := false,
+		depth := BSSGP_DECODE_DEPTH_L3
 	};
 	g_gb[1].cfg := {
 		nsei := 97,
@@ -299,7 +300,8 @@
 				},
 			cell_id := 20961
 		},
-		sgsn_role := false
+		sgsn_role := false,
+		depth := BSSGP_DECODE_DEPTH_L3
 	};
 	g_gb[2].cfg := {
 		nsei := 98,
@@ -312,7 +314,8 @@
 				},
 			cell_id := 20962
 		},
-		sgsn_role := false
+		sgsn_role := false,
+		depth := BSSGP_DECODE_DEPTH_L3
 	};
 
 	f_init_vty();