[web] robustify safaridriver launch sequence

[yjbanov]

Improve safaridriver launch sequence as follows:

• Fix retry: the previous version would call _startDriverProcess recursively from within a listener to the stderr output. But by then the outer _startDriverProcess call would have completed its future, so the retry mechanism would kick in while tests are already running.
• Wait for safaridriver server process to start listening and responding to WebDriver commands (using /status as the smoke test).
• Smoke-test the driver session by attempting to list all windows (WebDriver.windows).
• When safaridriver fails to pass all of the above checks, both close the session and kill the safaridriver process. Killing the process alone leaves Safari windows open. Closing the session alone leaves safaridriver processes open.
• When tests are finished use quit() instead of window.close(), because the latter does not close the session.

[flutter-dashboard]

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[stuartmorgan-g]

test-exempt: is a test

[vashworth]

LGTM

[eyebrowsoffire]

LGTM!