BSC_Tests: introduce f_mt_l3_transceive() sending BSSAP -> RSL

Change-Id: I5f1685815a4477b4d50351d3518ae21dd7d20139
Related: OS#3716
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index 544e79b..0632852 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -4484,6 +4484,23 @@
 	setverdict(pass);
 }
 
+private function f_mt_l3_transceive(template (present) RslLinkId link_id := tr_RslLinkID_DCCH(0),
+				    template (value) OCT1 dlci := '00'O,
+				    octetstring l3 := '0123456789'O)
+runs on MSC_ConnHdlr {
+	BSSAP.send(PDU_BSSAP:{
+		discriminator := '1'B,
+		spare := '0000000'B,
+		dlci := dlci,
+		lengthIndicator := lengthof(l3),
+		pdu := {
+			dtap := l3
+		}
+	});
+	RSL.receive(tr_RSL_DATA_REQ(g_chan_nr, link_id, l3));
+	setverdict(pass);
+}
+
 /* BSC asks for inter-BSC HO, but the MSC decides that it won't happen and
  * simply never sends a BSSMAP Handover Command. */
 private function f_tc_ho_out_fail_no_msc_response(charstring id) runs on MSC_ConnHdlr {