test: fix flaky test-http2-session-timeout

Increase server timeout, reduce frequency of calls and
unbind timeout after runs are done in order to avoid
race conditions. Temporarily moved to sequential.

Fixes: https://github.com/nodejs/node/issues/15326
PR-URL: https://github.com/nodejs/node/pull/15338


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>