Add more UI tests

[jtpio]

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: Add support for opening a document with a different factory #6315
• check Markdown is rendered correctly: Fix rendering of markdown #6318
• check the file name searcher can be added to the file browser toolbar via the settings: Update to JupyterLab 4.0.0a35 #6757
• check micro toolbars are not rendered for terminals and files: Hides 'micro' toolbars in notebook #6791
• snapshots for the file editor: added white space to bottom of edit files #6804

This replaces the issue opened in RetroLab: jupyterlab/retrolab#228