fix issue where PDCP SDU was not going through UE stack to SS

this cause the PDCP echo reply not to appear in MAC/PCAP

srsue/test/ttcn3/hdr/ttcn3_syssim.h

@@ -45,6 +45,7 @@ class ttcn3_syssim : public syssim_interface_phy,
                      public rrc_interface_rlc,
                      public rlc_interface_pdcp,
                      public rrc_interface_pdcp,
+                     public gw_interface_pdcp,
                      public srslte::pdu_queue::process_callback
 {
 public:

srsue/test/ttcn3/src/ttcn3_syssim.cc

@@ -124,7 +124,7 @@ int ttcn3_syssim::init(const all_args_t& args_)
   // Init SS layers
   pdus.init(this, log);
   rlc.init(&pdcp, this, &stack.timers, 0 /* RB_ID_SRB0 */);
-  pdcp.init(&rlc, this, nullptr);
+  pdcp.init(&rlc, this, this);
 
   return SRSLTE_SUCCESS;
 }

srsue/test/ttcn3/src/ttcn3_ue.cc

@@ -253,5 +253,5 @@ void ttcn3_ue::send_queued_data()
 void ttcn3_ue::loop_back_pdu_with_tft(uint32_t input_lcid, srslte::unique_byte_buffer_t pdu)
 {
   log.info_hex(pdu->msg, pdu->N_bytes, "Rx PDU (%d B) on lcid=%d, looping back\n", pdu->N_bytes, input_lcid);
-  stack->write_sdu(input_lcid, std::move(pdu), false);
+  stack->write_sdu(input_lcid, std::move(pdu), true);
 }