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/gadgets/config_relation.yaml

@@ -0,0 +1,36 @@
+# A belongsToMany relation with a manage form and a pivot form, and a
+# deferrable hasMany relation (nullable gadget_id) whose manage form lives
+# in the plugin's models directory ($/ path) and carries a fileupload and a
+# datepicker field. Both declare every toolbar button.
+members:
+    label: Members
+    view:
+        list:
+            columns:
+                email:
+                    label: Email
+        toolbarButtons: create|update|delete|link|unlink
+        showSearch: true
+    manage:
+        form: $/acme/deferred/models/member/fields.yaml
+        list:
+            columns:
+                email:
+                    label: Email
+        showSearch: true
+    pivot:
+        form: $/acme/deferred/models/member/pivot_fields.yaml
+parts:
+    label: Parts
+    view:
+        list:
+            columns:
+                label:
+                    label: Label
+        toolbarButtons: create|update|delete|link|unlink
+    manage:
+        form: $/acme/deferred/models/part/fields.yaml
+        list:
+            columns:
+                label:
+                    label: Label