selftests: mptcp: connect: add dedicated port counter

This patch adds a new dedicated counter 'port' instead of TEST_COUNT
to increase port numbers in mptcp_connect.sh.

This can avoid outputting discontinuous test counters.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20240308-upstream-net-next-20240308-selftests-mptcp-unification-v1-2-4f42c347b653@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>