test: increase buffer list coverage

1. test consuming BufferList which has strings with > 1 items
Refs: https://coverage.nodejs.org/coverage-0b6d3070a176d437/lib/internal/streams/buffer_list.js.html#L116

2. test consuming BufferList with > 1 items
Refs: https://coverage.nodejs.org/coverage-0b6d3070a176d437/lib/internal/streams/buffer_list.js.html#L152

PR-URL: https://github.com/nodejs/node/pull/36688


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>