bundles/apt: support spreading unattended-upgrades in a group

hooks/test_unattended_upgrades_spread.py

@@ -0,0 +1,24 @@
+from bundlewrap.exceptions import BundleError
+from bundlewrap.utils.text import bold, green, yellow
+from bundlewrap.utils.ui import io
+
+
+def test(repo, **kwargs):
+    for node in repo.nodes:
+        if not node.has_bundle('apt'):
+            continue
+
+        spread = node.metadata.get('apt/unattended-upgrades/spread_in_group', None)
+        if spread is None:
+            continue
+
+        for rnode in repo.nodes_in_group(spread):
+            rspread = rnode.metadata.get('apt/unattended-upgrades/spread_in_group', None)
+
+            if spread != rspread:
+                raise BundleError(f'{node.name} sets apt/unattended-upgrades/spread_in_group to "{spread}", but node {rnode.name} in that group does set "{rspread}"!')
+
+        io.stdout('{x} {node}  apt/unattended-upgrades/spread_in_group matches'.format(
+            x=green("✓"),
+            node=bold(node.name),
+        ))