I've found the Metabase test suite[0] to be very good considering it's real-world software written by a for-profit company. Coverage is good, the correct tests usually break when doing a refactor (stuff like "Oh, I thought this change was harmless but actually it breaks the permissions model"), etcetera. But the most important thing is that there's a strong team culture of a) demanding good tests on each PR b) hunting down flaky tests and other sources of friction.
Another neat thing was that there used to be a full-time SDET who spent a lot of time writing Cypress reproductions for known bugs. When you picked up the bug, you could un-skip the test that was right there waiting for you.
All that said, of course it's far from perfect!
0: https://github.com/metabase/metabase/ Backend unit tests are in test/, Frontend unit tests are in frontend/test, end-to-end tests (Cypress) are in e2e.
Another neat thing was that there used to be a full-time SDET who spent a lot of time writing Cypress reproductions for known bugs. When you picked up the bug, you could un-skip the test that was right there waiting for you.
All that said, of course it's far from perfect!
0: https://github.com/metabase/metabase/ Backend unit tests are in test/, Frontend unit tests are in frontend/test, end-to-end tests (Cypress) are in e2e.