In 539441, we've added chromedriver to package.json. It's v73, I'm assuming because that's the version we need in CI (because of Chromium version there probably not being up to date).
Unfortunately, now chromedriver doesn't work on a machine with a recent Chrome (v77), and my assumption is that would be the vast majority of developers.
Steps to reproduce on a macos with chromedriver previously installed via homebrew:
brew cask uninstall chromedriver cd mediawiki rm -rf node_modules npm install npm run selenium ... 2019-10-04T10:57:19.124Z ERROR @wdio/runner: Error: session not created: Chrome version must be between 70 and 73 (Driver info: chromedriver=73.0.3683.20 (8e2b610813e167eee3619ac4ce6e42e3ec622017),platform=Mac OS X 10.13.6 x86_64) ...