fix: slug in media paths (#793)

packages/core/test/data/config.mock.ts

@@ -1,11 +1,14 @@
 /* eslint-disable import/prefer-default-export */
+import { applyDefaults } from '@staticcms/core/actions/config';
+
 import type { BaseField, Config } from '@staticcms/core';
 
 export const createMockConfig = <EF extends BaseField>(
   options: Omit<Partial<Config<EF>>, 'collections'> & Pick<Config<EF>, 'collections'>,
-): Config<EF> => ({
-  backend: {
-    name: 'test-repo',
-  },
-  ...options,
-});
+): Config<EF> =>
+  applyDefaults({
+    backend: {
+      name: 'test-repo',
+    },
+    ...options,
+  });