selftests: netfilter: add test case for recent mismatch bug

Without 'nft_set_pipapo: fix incorrect avx2 match of 5th field octet"
this fails:

TEST: reported issues
  Add two elements, flush, re-add    1s  [ OK ]
  net,mac with reload                0s  [ OK ]
  net,port,proto                     3s  [ OK ]
  avx2 false match                   0s  [FAIL]
False match for fe80:dead:01fe:0a02:0b03:6007:8009:a001

Other tests do not detect the kernel bug as they only alter parts in
the /64 netmask.

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>