You can use Databricks Repos (https://docs.databricks.com/repos/index.html) specifically files in repos (https://docs.databricks.com/repos/work-with-notebooks-other-...) functionality that allows to use Python files (not notebooks!) as Python modules.

Another alternative is to split notebooks into “library notebooks” that just define transformations, and “orchestration notebooks” that use code library notebooks to execute a “business logic”.

In both approaches you can do code testing, etc.

P.S. I have a demo of both approaches here: https://github.com/alexott/databricks-nutter-repos-demo