fix length of frame generated by f_TC_rll_ud_ind

Because trxcon now requires frames of a minimum size some
BTS tests which send unpadded frames with a short payload
have started to fail.

Extend the payload size generated by f_TC_rll_ud_ind()
to make TC_rll_unit_data_ind_DCCH pass again.

Change-Id: Ibaa4124ebdec96623f48c38fac702e9bbd843869
Related: OS#3415
diff --git a/bts/BTS_Tests.ttcn b/bts/BTS_Tests.ttcn
index 941be28..04c4043 100644
--- a/bts/BTS_Tests.ttcn
+++ b/bts/BTS_Tests.ttcn
@@ -3602,7 +3602,7 @@
 	f_rslem_unregister(0, g_chan_nr);
 }
 testcase TC_rll_unit_data_ind_DCCH() runs on test_CT {
-	var octetstring l3 := f_rnd_octstring(15);
+	var octetstring l3 := f_rnd_octstring(20);
 	var RllTestCases tcs := {
 		valueof(t_EITC(0, valueof(ts_RslLinkID_DCCH(0)), l3, true)),
 		valueof(t_EITC(3, valueof(ts_RslLinkID_DCCH(3)), l3, true))