msc: f_tc_mt_t310: detect MNCC failure explicitly

Helped me find a failure cause: instead of T_guard timeout, immediately
show an unexpected MNCC event.

Related: SYS#5066
Change-Id: I49a15142a4b6c51ca767a884c0574f96e01d7cb1
diff --git a/msc/MSC_Tests.ttcn b/msc/MSC_Tests.ttcn
index d6e0aba..e6d270d 100644
--- a/msc/MSC_Tests.ttcn
+++ b/msc/MSC_Tests.ttcn
@@ -1804,6 +1804,7 @@
 	 * the MSC would stop T310.  However, the idea is to verify T310 expiration
 	 * here, so grab some popcorn and wait for MNCC DISC.ind. */
 	T310.start(30.0 + 2.0);
+	var MNCC_PDU mncc_rx;
 	alt {
 	[] T310.timeout {
 		setverdict(fail, "Timeout waiting for MNCC DISC.ind due to T310");
@@ -1814,6 +1815,11 @@
 		log("Rx MNCC DISC.ind, T310.read yelds ", T310.read);
 		setverdict(pass);
 		}
+	[] MNCC.receive(MNCC_PDU:?) -> value mncc_rx {
+		log("Rx ", mncc_rx);
+		setverdict(fail, "Expected MNCC DISC.ind, got some other MNCC message instead");
+		mtc.stop;
+	}
 	}
 
 	BSSAP.receive(tr_PDU_DTAP_MT(tr_ML3_MT_CC_DISC(cpars.transaction_id)));