Add priming packet to FIFO.

This forces the originating TNC to be out-of-sync when receiving frames.

test/util_data_channel_1.py

@@ -119,6 +119,11 @@ def t_highsnr_arq_short_station1(
     # Try to wait for the far-end to start up.
     # time.sleep(0.5)
 
+    # Add a frame to the TX queue at the beginning. This helps during testing so that
+    # the TNC is required to resend frames.
+    data = {"type": "ping", "command": "ping", "mycallsign": mycall, "dxcallsign": "BB5AA"}
+    sock.process_tnc_commands(json.dumps(data, indent=None))
+
     # Construct message to dxstation.
     b64_str = str(base64.b64encode(bytes(message, "UTF-8")), "UTF-8").strip()
     data = {