test(12.2-05): prove relation child and protected file scoping through the router

- acme.deferred fixture plugin over testdata/deferred (test-only), two
  controllers, recording Form and Relation hooks, two admins
- TestRelationChildScope*: every child route answers 404 for another
  parent, a hidden parent and another admin's pending child, changes
  nothing; undeclared toolbar buttons 403 before SQL; pivot whitelist
- TestProtectedFile*: foreign, pending and public files 404; only jpeg,
  png, gif and webp inline; nosniff, no-store and sandbox CSP everywhere

modules/cabana/testdata/deferred/controllers/locked/config_relation.yaml

@@ -0,0 +1,20 @@
+# The same relations with a reduced toolbar: parts may only be linked and
+# members only unlinked, so every other relation route answers 403.
+members:
+    label: Members
+    view:
+        list:
+            columns:
+                email:
+                    label: Email
+        toolbarButtons: unlink
+    pivot:
+        form: $/acme/deferred/models/member/pivot_fields.yaml
+parts:
+    label: Parts
+    view:
+        list:
+            columns:
+                label:
+                    label: Label
+        toolbarButtons: link