BSC_Tests: use correct payload types and encoding names on MGCP

The test currently use a hardcoded payload type and encoding name.
This does mean in practice that even when an assignment with EFR
is happeining. The MGCP responses to the BSC tell that the codec
is AMR. This is not correct. The testcases should always pick a
suitable payload type / encoding name in the MGCP response

- Add constants for IANA/3GPP assigned payload types
- Add function to lookup the right encoding name for a payload type
- Initalize the encoding name and payload type in g_media according
  to the BSSAP PDU.

Change-Id: I2735267091059e2f2169da80bdcd30abc2b1554b
Realted: OS#2728
diff --git a/library/MGCP_Types.ttcn b/library/MGCP_Types.ttcn
index 9d1a4e1..6bb266e 100644
--- a/library/MGCP_Types.ttcn
+++ b/library/MGCP_Types.ttcn
@@ -121,5 +121,16 @@
 	external function dec_MgcpMessage(in charstring  id) return MgcpMessage
 		with { extension "prototype(convert) decode(TEXT)" };
 
+	/* IANA / 3gpp assigned payload type numbers */
+	type enumerated SDP_FIELD_PayloadType {
+		PT_PCMU(0),
+		PT_GSM(3),
+		PT_PCMA(8),
+		PT_G729(18),
+		PT_GSMEFR(110),
+		PT_GSMHR(111),
+		PT_AMR(112),
+		PT_AMRWB(113)
+	}
 
 } with { encode "TEXT" }