selftests: netfilter: nft_concat_range.sh: add check for double-create bug

Add a test case for bug resolved with:
'netfilter: nft_set_pipapo_avx2: fix skip of expired entries'.

It passes on nf.git (it uses the generic/C version for insertion
duplicate check) but fails on unpatched nf-next if AVX2 is supported:

  cannot create same element twice      0s                        [FAIL]
Could create element twice in same transaction
table inet filter { # handle 8
[..]
  elements = { 1.2.3.4 . 1.2.4.1 counter packets 0 bytes 0,
               1.2.4.1 . 1.2.3.4 counter packets 0 bytes 0,
               1.2.3.4 . 1.2.4.1 counter packets 0 bytes 0,
               1.2.4.1 . 1.2.3.4 counter packets 0 bytes 0 }

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Florian Westphal <fw@strlen.de>