selftests: netfilter: conntrack_sctp_collision.sh: move to lib.sh infra

While at it, address warnings generated by shellcheck and fix following
minor issues:

- some distros place netem in 'extra' modules package, so add a skip check for netem-attach
failure.
- tc prints a warning for the 100mbit class:
"Warning: sch_htb: quantum of class 10001 is big. Consider r2q change."
Silence this by increasing the divisor.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-7-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>