selftests: mptcp: join: helper to filter TCP

This is more readable and reduces duplicated commands.

This might also be useful to add v6 support and switch to nftables.

Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>