MGCP_Test: use constants to predefine RTP payloads

The function f_TC_amr_x_x_rtp_conversion() is called with RTP payload
octetstrings in the parameter list. Lets define constants for those
octetstrings so that it is better visible which RTP payload is which.

Change-Id: I7efc22e2f8a941e36200dda7841089e6c97185c6

mgw/MGCP_Test.ttcn

@@ -2388,16 +2388,21 @@ module MGCP_Test {
 	 * functions are real world AMR RTP payloads including AMR header. The
 	 * payloads were extracted from a trace with known good payloads. */
 
+	const octetstring rtp_amr_5_90k_oa := '2014e959f35fdfe5e9667ffbc088818088'O;
+	const octetstring rtp_amr_5_90k_bwe := '217a567cd7f7f97a599ffef022206022'O;
+	const octetstring rtp_amr_5_15k_oa := '100c4e9ba850e30d5d53d04de41e7c'O;
+	const octetstring rtp_amr_5_15k_bwe := '10d3a6ea1438c35754f41379079f'O;
+
 	testcase TC_amr_oa_bwe_rtp_conversion() runs on dummy_CT {
-		f_TC_amr_x_x_rtp_conversion('2014e959f35fdfe5e9667ffbc088818088'O, '217a567cd7f7f97a599ffef022206022'O, "octet-align=1", "octet-align=0");
+		f_TC_amr_x_x_rtp_conversion(rtp_amr_5_90k_oa, rtp_amr_5_90k_bwe, "octet-align=1", "octet-align=0");
 	}
 
 	testcase TC_amr_oa_oa_rtp_conversion() runs on dummy_CT {
-		 f_TC_amr_x_x_rtp_conversion('100c4e9ba850e30d5d53d04de41e7c'O, '100c4e9ba850e30d5d53d04de41e7c'O, "octet-align=1", "octet-align=1");
+		 f_TC_amr_x_x_rtp_conversion(rtp_amr_5_15k_oa, rtp_amr_5_15k_oa, "octet-align=1", "octet-align=1");
 	}
 
 	testcase TC_amr_bwe_bwe_rtp_conversion() runs on dummy_CT {
-		 f_TC_amr_x_x_rtp_conversion('10d3a6ea1438c35754f41379079f'O, '10d3a6ea1438c35754f41379079f'O, "octet-align=0", "octet-align=0");
+		 f_TC_amr_x_x_rtp_conversion(rtp_amr_5_15k_bwe, rtp_amr_5_15k_bwe, "octet-align=0", "octet-align=0");
 	}
 
 	/* TODO: Double-DLCX (no retransmission) */