test: strictEqual() and RegExp in test-buffer-fill.js

Used assert.strictEqual() instead of assert.equal()
Passed a RegExp to assert.throws()
Broke lines to satisfy linting

PR-URL: https://github.com/nodejs/node/pull/9895


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