bsc: Submit valid l3 payload

OsmoBSC does some minimal parsing of l3 content to select MSC target,
match paging response to paging request, etc.
Since tests right now use potentially invalid data, osmo-bsc is not
rejecting conns providing invalid l3 content.

This commit makes sure TTCN3 tests pass valid l3 payloads to osmo-bsc,
so that they keep working once osmo-bsc starts rejecting invalid IEs it
parses.

Related: SYS#6280
Change-Id: I6e99ac39f32c9a981420b73f8d7d1568d2fa1c54
diff --git a/bsc/BSC_Tests_CBSP.ttcn b/bsc/BSC_Tests_CBSP.ttcn
index ae77ac8..3f1eff0 100644
--- a/bsc/BSC_Tests_CBSP.ttcn
+++ b/bsc/BSC_Tests_CBSP.ttcn
@@ -1005,7 +1005,7 @@
 	cell_list := ts_BSSMAP_CIL_CGI({bssmap_cgi(mp_cgi_bts0)});
 
 	/* first establish a dedicated channel */
-	var DchanTuple dt := f_est_dchan('23'O, 23, c_l3_payload);
+	var DchanTuple dt := f_est_dchan('23'O, 23, gen_l3_valid_payload());
 
 	/* then send ETWS PN */
 	f_cbsp_write_emerg(g_cbsp_msg_id, g_cbsp_ser_no, cell_list);