selftests: mptcp: add testcases for newly added addresses

This patch adds testcases to create subflows or signal addresses for the
newly added IPv4 or IPv6 addresses.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>