test: validate error message from buffer.equals

Adding regular expression as an argument to an assert.throws().

PR-URL: https://github.com/nodejs/node/pull/11215


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>