bsc: Wait for immedate assignment in f_chreq_act_ack

Sometimes TC_chan_rel_hard_rlsd_ms_dead could fail because the Immediate
assignment command would arrive in the RSL queue after it was cleared in
f_expect_chan_rel. The alt statement would now never complete since the
Immediate Assignment was blocking/hogging the queue. Wait explicitly for
the IMM ASS in f_chreq_act_ack before continuing.

Change-Id: I2831d4caf7f045b3396d28a978328e8a1097d8d3
diff --git a/bsc/BSC_Tests.ttcn b/bsc/BSC_Tests.ttcn
index 5af4573..e94b781 100644
--- a/bsc/BSC_Tests.ttcn
+++ b/bsc/BSC_Tests.ttcn
@@ -642,6 +642,7 @@
 	rx_rsl := f_exp_ipa_rx(0, tr_RSL_MsgTypeD(RSL_MT_CHAN_ACTIV));
 	var RslChannelNr chan_nr := rx_rsl.ies[0].body.chan_nr;
 	f_ipa_tx(0, ts_RSL_CHAN_ACT_ACK(chan_nr, fn+10));
+	rx_rsl := f_exp_ipa_rx(0, tr_RSL_IMM_ASSIGN(0));
 	return chan_nr;
 }