Check out storybook -> https://storybook.js.org/

People generally think of storybook as a place where you can view components in different configurations but you can reuse the “stories” for testing (in a kind of write once test everywhere kind of way)

I'd go ahead with cypress, both for e2e testing (whole app at once, component integration etc) and component testing (testing components in isolation)

I haven't used lit for more than testing viability testing where I am, but I've had success setting up jest with some vanilla is web components and doing unit testing that way.

I love Playwright. I've had better results (less flakiness) and better DX than with cypress. +1 for Playwright for me. Coupled with a CI that runs the tests and merge / publish only if everythings fine !

Give Web Test Runner a try

• runs in the browser - test in the same place you run
• extremely fast
• used by popular web components libraries like lit