Add unit tests

It is cumbersome to test the custom visual every time after a dependency update, thus it would be good to have automated unit tests that can verify if the custom visual still works with a default activity.

https://learn.microsoft.com/en-us/power-bi/developer/visuals/unit-tests-introduction

Start with the linked Microsoft Power BI unit-testing documentation, then inspect the repository to locate the custom visual and its current test setup; the issue names no files or entry points. Define a default activity case and verify that the visual still works automatically after dependency updates.

Written by the indexing model from the issue text.