selftests: forwarding: Test multipath tunneling

Add a GRE-tunneling test such that there are two tunnels involved, with
a multipath route listing both as next hops. Similarly to
router_multipath.sh, test that the distribution of traffic to the
tunnels honors the configured weights.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>