Add more UI tests

[jtpio]

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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