test: add test-buffer-prototype-inspect

lib/buffer.js defines Buffer.prototype.inspect() to override how buffers
are presented by util.inspect(). Add basic tests for it.

PR-URL: https://github.com/nodejs/node/pull/11600


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>