selftests: netfilter: nft_concat_range.sh: add check for flush+reload bug

selftests: netfilter: nft_concat_range.sh: add check for flush+reload bug

This test will fail without
the preceding commit ("netfilter: nft_set_pipapo_avx2: fix match retart if found element is expired"):

  reject overlapping range on add       0s                              [ OK ]
  reload with flush                 /dev/stdin:59:32-52: Error: Could not process rule: File exists
add element inet filter test { 10.0.0.29 . 10.0.2.29 }

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