MediaWiki core does have a basic set of Selenium based browser tests, but still lacks tests for the preview and diff functionalities. As an outcome of the incident T209012 (also see the umbrella ticket T208840) the need for such a test was identified by the WMDE-TechWish.
Full documentation of the incident at https://docs.google.com/document/d/1fEHquQx9CmGiWNCbok3OJKuP9sd41GMzm5iLSItVtYQ.