Optimize test-http2-large-file so it only allocates a single buffer.
PR-URL: https://github.com/nodejs/node/pull/26737
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Adrian Estrada <edsadr@gmail.com>