MSC_Tests: Reply to Classmark Req in TC_lu_imsi_auth_tmsi_encr_3_1_no_cm

After osmo-msc I73c7cb6a86624695bd9c0f59abb72e2fdc655131, osmo-msc
sends a BSSMAP Classmark Request if encounters a missing Classmark,
which is the case during LU when A5/3 is enabled.

Fix this test by answering the Classmark Request, if any.

Change-Id: I25578c050b7e105ed71b064891d4cd418ee30fcf
diff --git a/msc/MSC_Tests.ttcn b/msc/MSC_Tests.ttcn
index 4bdbc1d..93d7c77 100644
--- a/msc/MSC_Tests.ttcn
+++ b/msc/MSC_Tests.ttcn
@@ -1602,6 +1602,10 @@
 	}
 	f_mm_auth();
 	alt {
+	[] BSSAP.receive(tr_BSSMAP_ClassmarkReq) {
+		BSSAP.send(ts_BSSMAP_ClassmarkUpd(g_pars.cm2, g_pars.cm3));
+		repeat;
+		}
 	[] BSSAP.receive(tr_PDU_DTAP_MT(tr_ML3_MT_LU_Rej)) {
 		f_expect_clear();
 		}