ggsn: Allow receiving MTU ICMPv6 Option in RouterAdv

This Option is not mandatory, but open5gs-smfd started transmitting it
recently (ab64ff9514d8da121500be7b92c9d793ef20b894). Allow it.

Change-Id: Icf3e10dc599d41fb65051670fc3aec5ff0c67510
diff --git a/ggsn_tests/GGSN_Tests.ttcn b/ggsn_tests/GGSN_Tests.ttcn
index 118f1d5..435f71d 100644
--- a/ggsn_tests/GGSN_Tests.ttcn
+++ b/ggsn_tests/GGSN_Tests.ttcn
@@ -976,6 +976,16 @@
 		}
 	}
 
+	/* template for receiving/matching an ICMPv6 'MTU' option, rfc4861 4.6.4 */
+	template (present) OptionField tr_ICMP6_OptMTU(template (present) integer mtu := ?) := {
+		mTU := {
+			typeField := 5,
+			lengthIndicator := 1,
+			reserved := ?,
+			mTU_Value := mtu
+		}
+	}
+
 	/* template for receiving/matching an ICMPv6 router advertisement */
 	template (present) PDU_ICMPv6 tr_ICMPv6_RA(template (present) OCT16 prefix, template (present) INT1 prefix_len) := {
 		routerAdvertisement := {
@@ -989,9 +999,9 @@
 			routerLifetime := ?,
 			reachableTime := ?,
 			retransTimer := ?,
-			options := {
-				tr_ICMP6_OptPrefix(prefix, prefix_len)
-			}
+			options := ({ tr_ICMP6_OptPrefix(prefix, prefix_len) },
+				    { tr_ICMP6_OptPrefix(prefix, prefix_len), tr_ICMP6_OptMTU }
+				   )
 		}
 	}