selftests: mptcp: join: dump stats from history

In case of errors, dump the stats from history instead of using nstat.

There are multiple advantages to that:

- The same filters from pr_err_stats are used, e.g. the unused 'rate'
  column is not displayed.

- The counters are closer to the ones from when the test stopped.

- While at it, the errors can be better presented: error colours, a
  small indentation to distinguish the different parts, extra new lines.

Even if it should only happen in rare cases -- internal errors, or netns
issues -- if no history is available, 'nstat' is used like before, just
in case.

Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20251114-net-next-mptcp-sft-count-cache-stats-timeout-v1-4-863cb04e1b7b@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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