http2: add checks for server close callback

Verify that server close callbacks are being called

PR-URL: https://github.com/nodejs/node/pull/18182
Refs: https://github.com/nodejs/node/issues/18176


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>