test: refactor test-dgram-send-callback-multi-buffer

* use assert.strictEqual() instead of assert.equal()

PR-URL: https://github.com/nodejs/node/pull/9999


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