Update pcap pkt size to 1520 bytes

I see packets of 1520 bytes in the generated pcap running under the
docker setup. This happens when a lot of IPA concurrent clients send
messages and end up in the same TCP packet due to naggle algorithm.

Change-Id: I362371508ba83acc48376b6ed012a97a59d4b31b
diff --git a/ttcn3-dumpcap-start.sh b/ttcn3-dumpcap-start.sh
index eb390fd..a1e64fc 100755
--- a/ttcn3-dumpcap-start.sh
+++ b/ttcn3-dumpcap-start.sh
@@ -80,7 +80,7 @@
 	fi
 done
 
-$CMD -s 1500 -n ${ADDL_ARGS} -w "$TTCN3_PCAP_PATH/$TESTCASE.pcapng" >$TTCN3_PCAP_PATH/$TESTCASE.pcapng.stdout 2>&1 &
+$CMD -s 1520 -n ${ADDL_ARGS} -w "$TTCN3_PCAP_PATH/$TESTCASE.pcapng" >$TTCN3_PCAP_PATH/$TESTCASE.pcapng.stdout 2>&1 &
 PID=$!
 echo $PID > $PIDFILE_PCAP