test: validate errors in test-buffer-indexof

* validate errors in assert.throws
* use arrow functions

PR-URL: https://github.com/nodejs/node/pull/10752


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