test: make http2 timeout test robust

Instead of using magic values for the server timeout in
test-http2-session-timeout, measure the duration of the first request
(which should be longer than subsequent requests) and use that value.

Fixes: https://github.com/nodejs/node/issues/20628

PR-URL: https://github.com/nodejs/node/pull/24877
Fixes: https://github.com/https://github.com/nodejs/node/issues/20628


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>