Here's the full Quality Guideline for Shopware6 Plugins (and probably Apps).
The guidline is a bit cluttered in my opinion, so the relevant part for the automatic tests is a bit hidden in it.
Also for administration, Shopware itself is using Automated code tests with Cypress for JavaScript. I haven't seen many plugins having those tests. So I don't think it's mandatory (but tests are good thing to do in general).
Here's the full Quality Guideline for Shopware6 Plugins (and probably Apps).
The guidline is a bit cluttered in my opinion, so the relevant part for the automatic tests is a bit hidden in it.
Automatic code reviews with PhpStan and SonarQube are used, when uploading a Plugin to the Shopware store -> here's the Test Profile on GitHub.
Also for administration, Shopware itself is using Automated code tests with Cypress for JavaScript. I haven't seen many plugins having those tests. So I don't think it's mandatory (but tests are good thing to do in general).