test: increase Http2ServerResponse test coverage

Modify existing header tests for Http2ServerResponse to include
sendDate (get and set) and headersSent. Expand existing test
for end to include a check for closed. Add a new test for destroy.

PR-URL: https://github.com/nodejs/node/pull/15074
Refs: https://github.com/nodejs/node/issues/14985


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>