selftests: forwarding: Add missing multicast routing config entries

The two tests that make use of multicast routig (router.sh and
router_multicast.sh) are currently failing in the netdev CI because the
kernel is missing multicast routing support.

Fix by adding the required config entries.

Fixes: 6d4efada ("selftests: forwarding: Add multicast routing test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240208165538.1303021-1-idosch@nvidia.com


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