test: http2 emitGoAway post shutdown pre destroy

This commit tests use case when emitGoAway is called when client is
shutting down but is not destroyed.

Refs: https://github.com/nodejs/node/issues/14985
PR-URL: https://github.com/nodejs/node/pull/16215


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>