Last year I already wrote an article about how you could implement visual UI tests for your SharePoint solutions by using Puppeteer in combination with Jest. I still use these tools for setting up various UI tests in our products, but a couple of months ago, a tool called Cypress caught my attention.
A while ago I wrote an article about how you can quickly test and debug your SharePoint Framework code when it is already deployed on an environment. I use this technique quite a lot as I mostly work with application customizers. Info: Testing and debugging your SPFx solutions in production without causing any impact