test: net: add IP defragmentation duplicate-fragment regression test

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>