selftests: forwarding: Add test for dual VxLAN bridge

Configure VxLAN with an 802.1ad bridge and VxLAN with an 802.1d bridge
at the same time in same switch, verify that traffic passed as expected.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>