selftests: bonding: add test for passive LACP mode

selftests: bonding: add test for passive LACP mode

Add a selftest to verify bonding behavior when `lacp_active` is set to `off`.

The test checks the following:
- The passive LACP bond should not send LACPDUs before receiving a partner's
  LACPDU.
- The transmitted LACPDUs must not include the active flag.
- After transitioning to EXPIRED and DEFAULTED states, the passive side should
  still not initiate LACPDUs.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250815062000.22220-4-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>