selftests: mptcp: userspace: validate v4-v6 subflows mix

MPTCP protocol supports having subflows in both IPv4 and IPv6. In Linux,
it is possible to have that if the MPTCP socket has been created with
AF_INET6 family without the IPV6_V6ONLY option.

Here, a new IPv4 subflow is being added to the initial IPv6 connection,
then being removed using Netlink commands.

Cc: stable@vger.kernel.org # v5.19+
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>