selftests/net: add test for IP-in-IPv6 tunneling

commit 81c734dae203 ("ip6_tunnel: use skb_vlan_inet_prepare() in
__ip6_tnl_rcv()") was fine in and of itself, but its backport to 6.12
(and 6.6) broke IPv4-in-IPv6 tunneling, see [1]. This adds a self-test
for basic IPv4-in-IPv6 and IPv6-in-IPv6 functionality.

[1]: https://lore.kernel.org/all/CAA2RiuSnH_2xc+-W6EnFEG00XjS-dszMq61JEvRjcGS31CBw=g@mail.gmail.com/

Signed-off-by: Linus Heckemann <git@sphalerite.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Ricardo B. Marlière <rbm@suse.com>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260221114806.1231666-1-git@sphalerite.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>