selftests: mptcp: add test-cases for mixed v4/v6 subflows

Note that we can't guess the listener family anymore based on the client
target address: always use IPv6.

The fullmesh flag with endpoints from different families is also
validated here.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>