add a widget test to release process

RELEASE.md

@@ -99,7 +99,13 @@ If there is a network error during JS publish, run `npm run publish:all --skip-b
 Note that the use of `npm` instead of `jlpm` is [significant on Windows](https://github.com/jupyterlab/jupyterlab/issues/6733).
 
 At this point, run the `source scripts/release_test.sh` to test the wheel in
-a fresh conda environment with and without extensions installed.
+a fresh conda environment with and without extensions installed. Open and run
+the Outputs notebook and verify everything runs properly. Also add a cell with the following code and make sure the widget renders:
+
+```python
+from ipywidgets import IntSlider
+IntSlider()
+```
 
 ## Finish