selftests: netfilter: add test case for recent mismatch bug

Without 'netfilter: nf_set_pipapo: fix initial map fill' this fails:

TEST: reported issues
  Add two elements, flush, re-add       1s                              [ OK ]
  net,mac with reload                   1s                              [ OK ]
  net,port,proto                        1s                              [FAIL]
post-add: should have returned 10.5.8.0/24 . 51-60 . 6-17  but got table inet filter {
        set test {
                type ipv4_addr . inet_service . inet_proto
                flags interval,timeout
                elements = { 10.5.7.0/24 . 51-60 . 6-17 }
        }
}

The other sets defined in the selftest do not trigger this bug, it only
occurs if the first field group bitsize is smaller than the largest
group bitsize.

For each added element, check 'get' works and actually returns the
requested range.
After map has been filled, check all added ranges can still be
retrieved.

For each deleted element, check that 'get' fails.

Based on a reproducer script from Yi Chen.

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