selftests: nft_concat_range: add test for reload with no element add/del

Add a specific test for the reload issue fixed with
commit 23c54263 ("netfilter: nft_set_pipapo: allocate pcpu scratch maps on clone").

Add to set, then flush set content + restore without other add/remove in
the transaction.

On kernels before the fix, this test case fails:
  net,mac with reload    [FAIL]

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