test: improve reliability in http2-session-timeout

Use `setImmediate()` instead of `setTimeout()` to improve robustness of
test-http2-session-timeout.

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

PR-URL: https://github.com/nodejs/node/pull/22026


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>