test: improve assertion message in dgram test

PR-URL: https://github.com/nodejs/node/pull/16121
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>

test/parallel/test-dgram-send-callback-recursive.js

@@ -15,7 +15,7 @@ function onsend() {
   if (sent++ < limit) {
     client.send(chunk, 0, chunk.length, port, common.localhostIPv4, onsend);
   } else {
-    assert.strictEqual(async, true, 'Send should be asynchronous.');
+    assert.strictEqual(async, true);
   }
 }