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>

test/parallel/test-dgram-send-callback-multi-buffer.js

@@ -7,7 +7,7 @@ const dgram = require('dgram');
 const client = dgram.createSocket('udp4');
 
 const messageSent = common.mustCall(function messageSent(err, bytes) {
-  assert.equal(bytes, buf1.length + buf2.length);
+  assert.strictEqual(bytes, buf1.length + buf2.length);
 });
 
 const buf1 = Buffer.alloc(256, 'x');