selftests: forwarding: Add layer 2 miss test cases

Add test cases to verify that the bridge driver correctly marks layer 2
misses only when it should and that the flower classifier can match on
this metadata.

Example output:

 # ./tc_flower_l2_miss.sh
 TEST: L2 miss - Unicast                                             [ OK ]
 TEST: L2 miss - Multicast (IPv4)                                    [ OK ]
 TEST: L2 miss - Multicast (IPv6)                                    [ OK ]
 TEST: L2 miss - Link-local multicast (IPv4)                         [ OK ]
 TEST: L2 miss - Link-local multicast (IPv6)                         [ OK ]
 TEST: L2 miss - Broadcast                                           [ OK ]

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>