selftests: forwarding: Add a test for VXLAN symmetric routing

In a similar fashion to the asymmetric test, add a test for symmetric
routing. In symmetric routing both the ingress and egress VTEPs perform
routing in the overlay network into / from the VXLAN tunnel. Packets in
different directions use the same VNI - the L3 VNI. Different tenants
(VRFs) use different L3 VNIs.
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>