selftests: netfilter: nft_concat_range.sh: add check for overlap detection bug

selftests: netfilter: nft_concat_range.sh: add check for overlap detection bug

without 'netfilter: nft_set_pipapo: fix range overlap detection':

  reject overlapping range on add       0s         [FAIL]
Returned success for add { 1.2.3.4 . 1.2.4.1-1.2.4.2 } given set:
table inet filter {
	[..]
       elements = { 1.2.3.4 . 1.2.4.1 counter packets 0 bytes 0,
                    1.2.3.0-1.2.3.4 . 1.2.4.2 counter packets 0 bytes 0 }
}

The element collides with existing ones and was not added, but kernel
returned success to userspace.

Signed-off-by: Florian Westphal <fw@strlen.de>