selftests: bonding: add test for LACP actor port priority

Add comprehensive selftest to verify:
- Per-port actor priority setting via ad_actor_port_prio
- Aggregator selection behavior with port_priority ad_select policy

Also move cmd_jq helper from forwarding/lib.sh to net/lib.sh for
broader reusability across network selftests.

Here is the result output
  # ./bond_lacp_prio.sh
  TEST: bond 802.3ad (ad_actor_port_prio setting)                     [ OK ]
  TEST: bond 802.3ad (ad_actor_port_prio select)                      [ OK ]
  TEST: bond 802.3ad (ad_actor_port_prio switch)                      [ OK ]

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250902064501.360822-4-liuhangbin@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>