pcu: TC_paging_cs_from_sgsn_*: Fix race condition between BSSGP tx and RLCMAC rx

It can happen that the RLCMAC req arrives at osmo-pcu before the
CS_PAGING we send to it over BSSGP, in which case osmo-pcu will schedule
an RLCMAC DL Dummy Block. Take into account this scenario to avoid
failing id it occurs.

Change-Id: I30da93835c7738aefcd84691d83f39759dd4b599
diff --git a/pcu/PCU_Tests.ttcn b/pcu/PCU_Tests.ttcn
index 4b909c7..876b282 100644
--- a/pcu/PCU_Tests.ttcn
+++ b/pcu/PCU_Tests.ttcn
@@ -3627,8 +3627,16 @@
 		BSSGP[0].send(ts_BSSGP_CS_PAGING_IMSI(bvci, imsi));
 	}
 
-	/* Receive it on BTS side towards MS */
-	f_rx_rlcmac_dl_block_exp_pkt_pag_req(dl_block);
+	/* Now receive it on BTS side towards MS.
+	 * Skip any dummy blocks in case the PCUIF req arrives before the BSSP CS_PAGING:
+	 */
+	f_rx_rlcmac_dl_block_skip_dummy(dl_block, max_dummy := 10);
+
+	if (not match(dl_block, tr_RLCMAC_PACKET_PAG_REQ())) {
+		setverdict(fail, "Failed to match Packet Paging Request: ",
+			   dl_block, " vs ", tr_RLCMAC_PACKET_PAG_REQ());
+		f_shutdown(__BFILE__, __LINE__);
+	}
 
 	/* Make sure that Packet Paging Request contains the same P-TMSI/IMSI */
 	var PacketPagingReq req := dl_block.ctrl.payload.u.paging;