test: use default assertion messages

The string literal messages in addons-napi/test_buffer/test.js are less
helpful than the default messages, so use the default messages.

PR-URL: https://github.com/nodejs/node/pull/16808


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>