MGCP_Test: check payload type of received RTP packets

When an RTP packet is received, the payload type is not checked,
so we will not detect if the MGW emits packets with a wrong payload
type for some reason.

- Introduce a statistics counter that counts packets with wrong PT
- Update testcases so that they check for the statistics for wrong
  PT count.

Change-Id: I83d4b04656a16ced624024245a2fcb7a0ad48a8a
Related: OS#3384
diff --git a/library/RTP_Emulation.ttcn b/library/RTP_Emulation.ttcn
index 71cd8db..2a358a9 100644
--- a/library/RTP_Emulation.ttcn
+++ b/library/RTP_Emulation.ttcn
@@ -116,6 +116,8 @@
 	integer num_pkts_rx_err_seq,
 	/* number of packets received wrong timestamp */
 	integer num_pkts_rx_err_ts,
+	/* number of packets received wrong payload type */
+	integer num_pkts_rx_err_pt,
 	/* number of packets received during Rx disable */
 	integer num_pkts_rx_err_disabled
 }
@@ -127,6 +129,7 @@
 	bytes_payload_rx := 0,
 	num_pkts_rx_err_seq := 0,
 	num_pkts_rx_err_ts := 0,
+	num_pkts_rx_err_pt := 0,
 	num_pkts_rx_err_disabled := 0
 }
 
@@ -399,7 +402,11 @@
 		/* process received RTCP/RTP if receiver enabled */
 		[g_rx_enabled] RTP.receive(tr_rtp) -> value rx_rtp {
 			//log("RX RTP: ", rx_rtp);
+
 			/* increment counters */
+			if (rx_rtp.msg.rtp.payload_type != g_cfg.tx_payload_type) {
+				g_stats_rtp.num_pkts_rx_err_pt := g_stats_rtp.num_pkts_rx_err_pt+1;
+			}
 			g_stats_rtp.num_pkts_rx := g_stats_rtp.num_pkts_rx+1;
 			g_stats_rtp.bytes_payload_rx := g_stats_rtp.bytes_payload_rx +
 								lengthof(rx_rtp.msg.rtp.data);