selftests: forwarding: Add QinQ veto testing

Test that each veto that was added in the previous patch, is indeed
vetoed.

$ ./q_in_q_veto.sh

TEST: create 802.1ad vlan upper on top of a front panel             [ OK ]
TEST: create 802.1ad vlan upper on top of a bridge port             [ OK ]
TEST: create 802.1ad vlan upper on top of a lag                     [ OK ]
TEST: create 802.1ad vlan upper on top 802.1q bridge                [ OK ]
TEST: create 802.1ad vlan upper on top 802.1ad bridge               [ OK ]
TEST: create 802.1q vlan upper on top 802.1ad bridge                [ OK ]
TEST: create vlan upper on top of front panel enslaved to 802.1ad bridge
[ OK ]
TEST: create vlan upper on top of lag enslaved to 802.1ad bridge    [ OK ]
TEST: enslave front panel with vlan upper to 802.1ad bridge         [ OK ]
TEST: enslave lag with vlan upper to 802.1ad bridge                 [ OK ]
TEST: IP address addition to 802.1ad bridge                         [ OK ]
TEST: switch bridge protocol                                        [ OK ]

Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>