selftests: forwarding: Add VxLAN test with a VLAN-aware bridge

The test is very similar to its VLAN-unaware counterpart
(vxlan_bridge_1d.sh), but instead of using multiple VLAN-unaware
bridges, a single VLAN-aware bridge is used with multiple VLANs.

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