[test-runner] improve timeout handling #623

The current timeout system expects the browser to start within X milliseconds, and tests to finish within X milliseconds. This is a problem when you have especially long running tests, users need to configure very large timeouts which in turn lead to a bad experience when trying to debug failures.

Now that web dev server's message system is based on web sockets we can adopt the same inside the test runner. The web socket can be used by the test runner server to ping the browser to see if it's still alive, and we can configure a no response timeout for this ping.