Add function method to aggregate areas

This is standalone for now - we'll plumb it in later.

tests/app/broadcast_areas/test_utils.py

@@ -0,0 +1,71 @@
+import pytest
+
+from app.broadcast_areas.utils import aggregate_areas
+from app.models.broadcast_message import BroadcastMessage
+from tests import broadcast_message_json
+from tests.app.broadcast_areas.custom_polygons import (
+    BRISTOL,
+    BURFORD,
+    CHELTENHAM,
+    SANTA_A,
+    SKYE,
+)
+
+
+@pytest.mark.parametrize(('area_ids', 'expected_area_names'), [
+    (
+        [
+            'wd20-E05004294',  # Hester’s Way, Cheltenham (electoral ward)
+            'wd20-E05010981',  # Painswick & Upton, Stroud (electoral ward)
+        ], [
+            'Cheltenham',  # in Gloucestershire (upper tier authority)
+            'Stroud',  # in Gloucestershire
+        ],
+    ),
+    (
+        [
+            'wd20-E05004294',  # Hester’s Way, Cheltenham (electoral ward)
+            'wd20-E05009372',  # Hackney Central (electoral ward)
+        ], [
+            'Cheltenham',  # in Gloucestershire (upper tier authority)
+            'Hackney',  # in Greater London* (DB doesn't know this)
+        ],
+    ),
+    (
+        [
+            'lad20-E07000037',  # High Peak (lower tier authority)
+        ],
+        [
+            'High Peak',  # in Derbyshire (upper tier authority)
+        ]
+    ),
+    (
+        [
+            'lad20-E07000037',  # High Peak (lower tier authority)
+            'lad20-E07000035',  # Derbyshire Dales (lower tier authority)
+        ],
+        [
+            'Derbyshire Dales',  # in Derbyshire (upper tier authority)
+            'High Peak',  # in Derbyshire
+        ]
+    ),
+    (
+        [
+            'ctry19-E92000001',  # England
+        ],
+        [
+            'England',
+        ]
+    )
+])
+def test_aggregate_areas(
+    area_ids,
+    expected_area_names,
+):
+    broadcast_message = BroadcastMessage(
+        broadcast_message_json(area_ids=area_ids)
+    )
+
+    assert sorted(
+        area.name for area in aggregate_areas(broadcast_message.areas)
+    ) == expected_area_names