http2: remove unused onTimeout, add timeout tests

Remove unused onTimeout on Http2Session and Http2Stream because
the correct _onTimeout is already declared and in use. Expand
timeout tests to handle edge cases and additional arguments.

PR-URL: https://github.com/nodejs/node/pull/15539


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>