selftests: mptcp: simult_flows: unify errors msgs

selftests: mptcp: simult_flows: unify errors msgs

In order to unify what is printed in case of error, similar to what is
done in mptcp_connect.sh and mptcp_join.sh, it is interesting to do the
following modifications in simult_flows.sh:

- Print the rc errors at the end of the line.

- Print the MIB counters.

- Use the same ss options: add -M (MPTCP sockets) and -e (detailed
  socket information).

While at it, also print of the 'max' time only in case of success,
because 'mptcp_connect.c' will already print this info in case of error,
e.g.:

  transfer slower than expected! runtime 11948 ms, expected 11921 ms

Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250114-net-next-mptcp-st-more-debug-err-v1-1-2ffb16a6cf35@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>