test: improve buffer coverage

PR-URL: https://github.com/nodejs/node/pull/38538


Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>