msc: fix f_tc_mt_crcx_ran_reject(): properly handle Iu-ReleaseCommand

Previous commit [1] uncovers a problem in f_tc_mt_crcx_ran_reject():
this function uses as_clear_cmd_compl_disc(), which is expecting
A-interface (GERAN) specific BSSMAP Clear Command.  The Iu-interface
(UTRAN) specific RANAP Iu-ReleaseCommand is not handled at all.

The testcase was passing so far due to a bug in as_optional_cc_rel(),
which would unblock the alt-statemtnt on receipt of CC RELEASE, so
that we would never respond to RANAP Iu-ReleaseCommand.

Let's derive a new altstep from f_expect_clear() and use it.

Change-Id: Idd679bbf720a56a76cf37ab414b1e6d90e53278b
Related: [1] I0143b4d33b1ebe4cce99c09018540524c4626eec
diff --git a/msc/MSC_Tests.ttcn b/msc/MSC_Tests.ttcn
index 7e8f467..101d632 100644
--- a/msc/MSC_Tests.ttcn
+++ b/msc/MSC_Tests.ttcn
@@ -1752,8 +1752,8 @@
 		repeat;
 		}
 	[] MGCP.receive { repeat; }
-	[] as_clear_cmd_compl_disc();
-	[] as_optional_cc_rel(cpars);
+	[] as_optional_cc_rel(cpars); /* repeats internally */
+	[] as_expect_clear() { setverdict(pass); }
 	}
 }
 testcase TC_mt_crcx_ran_reject() runs on MTC_CT {