test: fix flaky http2-session-unref

It's possible for the connections to take too long and since the server
is already unrefed, the process will just exit. Instead adjust the test
so that server unref only happens after all sessions have been
successfuly established and unrefed. That still tests the same condition
but will not fail under load.

Backport-PR-URL: https://github.com/nodejs/node/pull/22850
PR-URL: https://github.com/nodejs/node/pull/20772
Fixes: https://github.com/nodejs/node/issues/20705
Fixes: https://github.com/nodejs/node/issues/20750
Fixes: https://github.com/nodejs/node/issues/20850


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>