selftests: mptcp: check subflow errors in close events

selftests: mptcp: check subflow errors in close events

This validates the previous commit: subflow closed events should contain
an error field when a subflow got closed with an error, e.g. reset or
timeout.

For this test, the chk_evt_nr helper has been extended to check
attributes in the matched events.

In this test, the 2 subflow closed events should have an error.

The 'Fixes' tag here below is the same as the one from the previous
commit: this patch here is not fixing anything wrong in the selftests,
but it validates the previous fix for an issue introduced by this commit
ID.

Fixes: 15cc10453398 ("mptcp: deliver ssk errors to msk")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260127-net-mptcp-dup-nl-events-v1-4-7f71e1bc4feb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>