Nick Halsey made a usability test on Twenty Seventeen. See the results here. The method seems to be observation of usage and notes during the test. Four users tested the new theme. According to the web guru Nielsen you could find around 90% of the usability errors in this way.
Multimusen.dk will set a few cookies from Doubleclick, Google and the Social Media plugins they ay set some cookies. Some of my pages use APIs - such as YouTube, LinkedIn, Google Fonts, Google Maps, Mapbox, Spotify, Jetpack, Twitter, Facebook &c.. Such plugins may set the odd cookie.