In my Vaadin projects, I usually use Karibu Testing for unit and/or Vaadins TestBench for end-to-end tests. Karibu Testing is an open source testing framework developed by Martin Vysny and TestBench the commercial UI testing framework based on Selenium.
Testing with TestBench/Selenium is often slow and flaky. I encountered non-reproducible problems when running the test. So I was looking for an alternative.