Add more UI tests

We should add a couple of new UI tests, ideally alongside changes in PRs.

This issue tracks some checks we should add to:

- [ ] check support for other factories: https://github.com/jupyter/notebook/pull/6315
- [x] check Markdown is rendered correctly: https://github.com/jupyter/notebook/pull/6318
- [ ] check the file name searcher can be added to the file browser toolbar via the settings: https://github.com/jupyter/notebook/pull/6757
- [ ] check micro toolbars are not rendered for terminals and files: https://github.com/jupyter/notebook/pull/6791
- [ ] snapshots for the file editor: https://github.com/jupyter/notebook/pull/6804

This replaces the issue opened in RetroLab: https://github.com/jupyterlab/retrolab/issues/228

Review the open checklist items and their referenced pull requests, especially PRs 6315, 6757, 6791, and 6804, then locate the existing UI test patterns in the repository. The work is done when the remaining factory, file-browser toolbar, micro-toolbar, and file-editor snapshot checks are covered; the Markdown check is already marked complete.

Written by the indexing model from the issue text.