selftests: mptcp: check no dup close events after error

selftests: mptcp: check no dup close events after error

This validates the previous commit: subflow closed events are re-sent
with less info when the initial subflow is disconnected after an error
and each time a subflow is closed after that.

In this new test, the userspace PM is involved because that's how it was
discovered, but it is not specific to it. The initial subflow is
terminated with a RESET, and that will cause the subflow disconnect.
Then, a new subflow is initiated, but also got rejected, which cause a
second subflow closed event, but not a third one.

While at it, in case of failure to get the expected amount of events,
the events are printed.

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: d82809b6c5f2 ("mptcp: avoid duplicated SUB_CLOSED events")
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-v1-2-7f71e1bc4feb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>