selftests: netfilter: Torture nftables netdev hooks

Add a ruleset which binds to various interface names via netdev-family
chains and flowtables and massage the notifiers by frequently renaming
interfaces to match these names. While doing so:
- Keep an 'nft monitor' running in background to receive the notifications
- Loop over 'nft list ruleset' to exercise ruleset dump codepath
- Have iperf running so the involved chains/flowtables see traffic

If supported, also test interface wildcard support separately by
creating a flowtable with 'wild*' interface spec and quickly add/remove
matching dummy interfaces.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>