Clean up unit tests

[tkrabel-db]

Follow up for #389. It makes sense cleaning up the unit tests in test_notebook_document.py and test_language_server.py, e.g. wrt shared objects.
CC @ccordoba12

[ccordoba12]

Please try to address this after PR #389 is merged so we don't have the tests in those two files growing apart too much.

[tkrabel-db]

Will do.