selftests: netfilter: avoid test timeouts on debug kernels

Jakub reports that some tests fail on netdev CI when executed in a debug
kernel.

Increase test timeout to 30m, this should hopefully be enough.
Also reduce test duration where possible for "slow" machines.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240429105736.22677-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: netfilter: nft_nat_zones.sh: shellcheck cleanups

While at it: No need for iperf here, use socat.
This also reduces the script runtime.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240418152744.15105-8-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

selftests: netfilter: move to net subdir

.. so this can start re-using existing lib.sh infra in next patches.

Several of these scripts will not work, e.g. because they assume
rp_filter is disabled, or reliance on a particular version/flavor
of "netcat" tool.

Add config settings for them.

nft_trans_stress.sh script is removed, it also exists in the nftables
userspace selftests. I do not see a reason to keep two versions in
different repositories/projects.

The settings file is removed for now:

It was used to increase the timeout to avoid slow scripts from getting
zapped by the 45s timeout, but some of the slow scripts can be sped up.
Re-add it later for scripts that cannot be sped up easily.

Update MAINTAINERS to reflect that future updates to netfilter
scripts should go through netfilter-devel@.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-2-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>