Merge branch 'selftests-mptcp-share-code-and-fix-shellcheck-warnings'

Matthieu Baerts says:

====================
selftests: mptcp: share code and fix shellcheck warnings

This series cleans MPTCP selftests code.

Patch 1 stops using 'iptables-legacy' if available, but uses 'iptables',
which is likely 'iptables-nft' behind.

Patches 2, 4 and 6 move duplicated code to mptcp_lib.sh. Patch 3 is a
preparation for patch 4, and patch 5 adds generic actions at the
creation and deletion of netns.

Patches 7 to 11 disable a few shellcheck warnings, and fix the rest, so
it is easy to spot real issues later. MPTCP CI is checking that now.

Patch 12 avoids redoing some actions at init time twice, e.g. restarting
the pm events tool.

v1: https://lore.kernel.org/r/20240305-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v1-0-66618ea5504e@kernel.org
====================

Link: https://lore.kernel.org/r/20240306-upstream-net-next-20240304-selftests-mptcp-shared-code-shellcheck-v2-0-bc79e6e5e6a0@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>