selftests: forwarding: Add a test for FDB activity notification control

selftests: forwarding: Add a test for FDB activity notification control

Test various aspects of FDB activity notification control:

* Transitioning of an FDB entry from inactive to active state.

* Transitioning of an FDB entry from active to inactive state.

* Avoiding the resetting of an FDB entry's last activity time (i.e.,
  "updated" time) using the "norefresh" keyword.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20250812071810.312346-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>