SGSN_Tests: Introduce NUM_GB to avoid '3' as magic value

Change-Id: I28b7dd26c5c129ec41c6349de9ddc9ef0ffee8ac
diff --git a/sgsn/SGSN_Tests.ttcn b/sgsn/SGSN_Tests.ttcn
index d87c09d..6665dc0 100644
--- a/sgsn/SGSN_Tests.ttcn
+++ b/sgsn/SGSN_Tests.ttcn
@@ -84,9 +84,10 @@
 	BssgpConfig cfg
 };
 
-type record length(3) of GbInstance GbInstances;
-type record length(3) of NSConfiguration NSConfigurations;
-type record length(3) of BssgpCellId BssgpCellIds;
+const integer NUM_GB := 3;
+type record length(NUM_GB) of GbInstance GbInstances;
+type record length(NUM_GB) of NSConfiguration NSConfigurations;
+type record length(NUM_GB) of BssgpCellId BssgpCellIds;
 
 type component test_CT {
 	var GbInstances g_gb;