selftests: netfilter: Introduce tests for sets with range concatenation

This test covers functionality and stability of the newly added
nftables set implementation supporting concatenation of ranged
fields.

For some selected set expression types, test:
- correctness, by checking that packets match or don't
- concurrency, by attempting races between insertion, deletion, lookup
- timeout feature, checking that packets don't match expired entries

and (roughly) estimate matching rates, comparing to baselines for
simple drop on netdev ingress hook and for hash and rbtrees sets.

In order to send packets, this needs one of sendip, netcat or bash.
To flood with traffic, iperf3, iperf and netperf are supported. For
performance measurements, this relies on the sample pktgen script
pktgen_bench_xmit_mode_netif_receive.sh.

If none of the tools suitable for a given test are available, specific
tests will be skipped.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>