test: improve reliability of http2-session-timeout

Check actual expired time rather than relying on a number of calls to
setTimeout() in test-http2-session-timeout more robust.

Backport-PR-URL: https://github.com/nodejs/node/pull/22850
PR-URL: https://github.com/nodejs/node/pull/20692
Fixes: https://github.com/nodejs/node/issues/20628


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>