selftests: forwarding: Add multipath tunneling nexthop test

Add a nexthop objects version of gre_multipath.sh. Unlike the original
test, it also tests IPv6 overlay which is not possible with the legacy
nexthop implementation. See commit 9a2ad362 ("selftests: forwarding:
gre_multipath: Drop IPv6 tests") for more info.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>