Add unit tests for _beacons and wire into CI

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).

salt/_beacons/postgres_pillar_beacon.py

@@ -83,7 +83,7 @@ def _query(sql):
     return result.stdout
 
 
-def beacon(config):
+def beacon(config):  # noqa: C901
     retval = []
 
     watermark = _read_watermark()