selftests: mptcp: add explicit check for new mibs

Instead of duplicating the all existing TX check with
the TX side, add the new ones on selected test cases.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>