add a skeleton for route test pages

Co-authored-by: Patrik Oldsberg <poldsberg@gmail.com>
Co-authored-by: Johan Haals <johan.haals@gmail.com>
Signed-off-by: Fredrik Adelöw <freben@gmail.com>

packages/frontend-app-api/src/createApp.tsx

@@ -92,12 +92,16 @@ export function createApp(options: { plugins: BackstagePlugin[] }): {
       ]),
     );
 
-    return createExtensionInstance({
+    const newInstance = createExtensionInstance({
       extension: instanceParams.extension,
       source: instanceParams.source,
       config: instanceParams.config,
       attachments,
     });
+
+    instances.set(instanceParams.extension.id, newInstance);
+
+    return newInstance;
   }
 
   const rootConfigs = attachmentMap.get('root')?.get('default') ?? [];