test(editor): reduce flaky in turbo renderer test (#11973)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit

- **Tests**
  - Improved test resource cleanup for the viewport turbo renderer, ensuring proper teardown after each test for more reliable and isolated test execution.
  - Enhanced stability by preventing errors during cleanup when certain elements or editors are absent.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/integration-test/src/__tests__/utils/setup.ts

@@ -129,13 +129,13 @@ export async function setupEditor(
   const appElement = await createEditor(collection, mode, extensions);
 
   return () => {
-    appElement.remove();
+    appElement?.remove();
     cleanup();
   };
 }
 
 export function cleanup() {
-  window.editor.remove();
+  window.editor?.remove();
 
   delete (window as any).collection;