test: improve internal/buffer.js test coverage

Added tests buffer.js methods to write 48 bit value to
improve test coverage.

PR-URL: https://github.com/nodejs/node/pull/21061


Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Lance Ball <lball@redhat.com>