selftests: forwarding: Test action VLAN modify

Construct a basic topology consisting of two hosts connected using a
VLAN-aware bridge. Put each port in a different VLAN and test that ping
fails.

Add ingress and egress filters with a VLAN modify action and test that
ping passes.

Signed-off-by: Danielle Ratson <danieller@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>