selftests: net: Fix bridge backup port test flakiness

The test toggles the carrier of a bridge port in order to test the
bridge backup port feature.

Due to the linkwatch delayed work the carrier change is not always
reflected fast enough to the bridge driver and packets are not forwarded
as the test expects, resulting in failures [1].

Fix by busy waiting on the bridge port state until it changes to the
desired state following the carrier change.

[1]
 # Backup port
 # -----------
 [...]
 # TEST: swp1 carrier off                                              [ OK ]
 # TEST: No forwarding out of swp1                                     [FAIL]
 [  641.995910] br0: port 1(swp1) entered disabled state
 # TEST: No forwarding out of vx0                                      [ OK ]

Fixes: b4084530 ("selftests: net: Add bridge backup port and backup nexthop ID test")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://lore.kernel.org/r/20240208123110.1063930-1-idosch@nvidia.com


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