Add 100%-coverage unit tests for the three custom salt beacons
(postgres_pillar_beacon, rules_beacon, zeek) and add salt/_beacons to
the python-test workflow's paths trigger and matrix.
To pass the workflow's flake8 lint over the whole directory:
- zeek.py: reindent to 4 spaces, drop trailing blank line, noqa the
Salt-injected __salt__ references (F821); no logic change.
- postgres_pillar_beacon.py: noqa C901 on beacon() (complexity 13 > 12).