selftests: xsk: Add single packet test

Add a test where a single packet is sent and received. This might
sound like a silly test, but since many of the interfaces in xsk are
batched, it is important to be able to validate that we did not break
something as fundamental as just receiving single packets, instead of
batches of packets at high speed.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210922075613.12186-12-magnus.karlsson@gmail.com