selftests: mptcp: add IPv4-mapped IPv6 testcases

Here, we make sure we support IPv4-mapped in IPv6 addresses in different
contexts:

- a v4-mapped address is received by the PM and can be used as v4.
- a v4 address is received by the PM and can be used even with a v4
  mapped socket.

We also make sure we don't try to establish subflows between v4 and v6
addresses, e.g. if a real v6 address ends with a valid v4 address.

Co-developed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
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>