Add a unit test for the IP datagram reassembler (CONFIG_IP_DEFRAG) that covers the duplicate-last-fragment scenario. Without the fix the last fragment will re-trigger datagram delivery, increasing udp_rx_count to 2 and effectively failing the test; with it applied the test passes with udp_rx_count == 1. Signed-off-by: Mateusz Furdyna <mateusz.furdyna@nokia.com> Reviewed-by: Simon Glass <sjg@chromium.org>