Fix ui-tests

[brichet]

The current project structure includes ui-tests as a workspace of the project.
In the same time there is an empty yarn.lock file it the ui-test directory, meaning (for yarn) that it should be treated as a completely separated project.

This PR assumes that we should treat the ui-tests as a separated project, and remove it from the project workspaces. It also allows to modify its yarn.lock file during CI.

If we aim to treat the ui-tests as a workspace (instead of a separated project), we should probably fix the check_release test which fails with workspaces (e.g. https://github.com/QuantStack/jupyter-sql-cell/actions/runs/6198336848/job/16828578630)

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[fcollonval]

FYI the releaser test will be fixed by jupyter-server/jupyter_releaser#522

[fcollonval]

Thanks @brichet

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