selftests: net: Test bridge backup port when port is administratively down

Test that packets are redirected to the backup port when the primary
port is administratively down.

With the previous patch:

 # ./test_bridge_backup_port.sh
 [...]
 TEST: swp1 administratively down                                    [ OK ]
 TEST: No forwarding out of swp1                                     [ OK ]
 TEST: Forwarding out of vx0                                         [ OK ]
 TEST: swp1 administratively up                                      [ OK ]
 TEST: Forwarding out of swp1                                        [ OK ]
 TEST: No forwarding out of vx0                                      [ OK ]
 [...]
 Tests passed:  89
 Tests failed:   0

Without the previous patch:

 # ./test_bridge_backup_port.sh
 [...]
 TEST: swp1 administratively down                                    [ OK ]
 TEST: No forwarding out of swp1                                     [ OK ]
 TEST: Forwarding out of vx0                                         [FAIL]
 TEST: swp1 administratively up                                      [ OK ]
 TEST: Forwarding out of swp1                                        [ OK ]
 [...]
 Tests passed:  85
 Tests failed:   4

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250812080213.325298-3-idosch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>