selftests: forwarding: Add PVID test case for VXLAN with VLAN-aware bridges

When using VLAN-aware bridges with VXLAN, the VLAN that is mapped to the
VNI of the VXLAN device is that which is configured as "pvid untagged"
on the corresponding bridge port.

When these flags are toggled or when the VLAN is deleted entirely,
remote hosts should not be able to receive packets from the VTEP.

Add a test case for above mentioned scenarios.

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