Merge branch 'selftests-mptcp-counter-cache-stats-before-timeout'

Matthieu Baerts says:

====================
selftests: mptcp: counter cache & stats before timeout

Here are a bunch of small improvements to the MPTCP selftests:

- Patch 1: move code to mptcp_lib.sh to prepare the new features.

- Patch 2: simplify mptcp_lib_pr_err_stats helper use.

- Patch 3: remove unused last column from nstat output.

- Patch 4: improve stats dump in mptcp_join.sh.

- Patch 5: get counters from nstat history and simplify mptcp_connect.sh.

- Patch 6: avoid taking the same packet trace twice.

- Patch 7: wait for an event instead of a fix time.

- Patch 8: instead of using 'timeout' and print the stats after, another
  internal timeout is used: if it fires, it will print stats, then stop
  everything. This avoids confusions around stats in case of timeout.
====================

Link: https://patch.msgid.link/20251114-net-next-mptcp-sft-count-cache-stats-timeout-v1-0-863cb04e1b7b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>