buffer, test: add tests for buffer.toJSON

* Pull out the tests for buffer.toJSON from test-buffer-alloc
* Add tests for serializing a 0-length buffer

PR-URL: https://github.com/nodejs/node/pull/10979


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>