selftests: mptcp: join: validate event numbers

This test extends "delete and re-add" and "delete re-add signal" to
validate the previous commit: the number of MPTCP events are checked to
make sure there are no duplicated or unexpected ones.

A new helper has been introduced to easily check these events. The
missing events have been added to the lib.

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: b911c97c ("mptcp: add netlink event support")
Cc: stable@vger.kernel.org
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>