selftests: forwarding: Add IPv6 GRE remote change tests

Test that after changing the remote address of an ip6gre net device
traffic is forwarded as expected. Test with both flat and hierarchical
topologies and with and without an input / output keys.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/02b05246d2cdada0cf2fccffc0faa8a424d0f51b.1729866134.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>