pcu: Fix race condition in TC_t3141

The test was expecting to always receive at least 1 DL packet with
USF_UNUSED, but since we implemented idle blocks in PCUIF that may not
always be the case anymore. If the TBF is freed in between last
requested block and next one, there may be no TBF and hence no MS
listening on the TS, so PCU will optimize and send an idle block.

Change-Id: Ia301c01a3f5c3fd0b11d8f20e39061aa7abc6127
diff --git a/pcu/PCU_Tests.ttcn b/pcu/PCU_Tests.ttcn
index 7fcab4a..01113ab 100644
--- a/pcu/PCU_Tests.ttcn
+++ b/pcu/PCU_Tests.ttcn
@@ -1428,6 +1428,15 @@
 					  block_nr := nr.blk_nr));
 		repeat;
 	}
+	[ul_tbf_usf_req] BTS.receive(tr_PCUIF_DATA_PDTCH(nr.bts_nr,
+					   tr_PCUIF_DATA(nr.trx_nr, nr.ts_nr, sapi := PCU_IF_SAPI_PDTCH),
+					   omit)) {
+			/* TBF was dropped by T3141, and PCU answered with an IDLE block to
+			   our last RTS.req because there's no longer any MS listening on
+			   the TS. */
+		   setverdict(pass);
+		   break;
+	}
 	[] T_3141.timeout {
 		log("T_3141 expired but TBF is still active, unexpected");
 		f_shutdown(__BFILE__, __LINE__);