mme: TC_s1ap_tau_unknown_guti: expect UeContextReleaseCmd

The MME is sending the message after TAU Reject, and it may be forwarded
through TTCN3 components while some of them are already being torn down
since the test is finished.

Change-Id: I6b915d0e32a46fa9546b3b1b96aa69ad43baae9e

mme/MME_Tests.ttcn

@@ -1297,6 +1297,7 @@ private function f_TC_tau_unknown_guti(ConnHdlrPars pars) runs on ConnHdlr {
 		}
 	}
 
+	as_s1ap_handle_UeContextReleaseCmd();
 }
 testcase TC_s1ap_tau_unknown_guti() runs on MTC_CT {
 	var charstring id := testcasename();