[wptrunner] Stop the runner process before the browser (#48030)
... via a stop message that tells the runner to gracefully tear down
the protocol.
- For WebDriver-based executors, this will close the session
appropriately 1. - For
chromedriverin particular, this will ensure some per-session
/tmp/.org.chromium.Chromium.*temporary directories 2, which are
currently leaked, are deleted.
Also, remove the redundant runner_teardown command from the
runner to main process.