Add support for headless runs of any browser.
This adds an optional flag, which defaults to false. When set to true, the
BROWSER_HEADLESS environment variable is set to
1, causing webdriver to request a headless browser.
This ties in with https://github.com/moodlehq/moodle-ci-runner/pull/43 and MDL-66979. This change can land independently to the other issues.