Add selenium to GitHub actions tests

[datapythonista]

Tests on the asv generated website are implemented with Selenium. Currently, those tests are skipped in GitHub actions, since no selenium binary is selected. Enabling the tests is as easy as providing --webdriver=ChromeHeadless to the pytest call. Selenium/Chrome seem to be available in GitHub actions workers without having to perform anything. But for whatever reason, Selenium tests seem to be working fine in Appveyor, but they are failing in GitHub actions, as the elements the tests look for in the generated website aren't found (I couldn't reproduce the error locally, seem to be something with GitHub actions).