test: expand http2 util test coverage for headers

Expand existing test cases to hit some additional branches of
toHeaderObject and mapToHeaders.

PR-URL: https://github.com/nodejs/node/pull/15493


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>