selftests: xsk: Add test case for packets at end of UMEM

Add test case to testapp_invalid_desc for valid packets at the end of
the UMEM.

Signed-off-by: Kal Conley <kal.conley@dectris.com>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/r/20230403145047.33065-3-kal.conley@dectris.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>