test: improve assert message for debugging

Previously, the assertion changed in this commit would print the same
message no matter which of the four test cases failed. The assert has
been changed so that it will indicate which test case failed.

Refs: nodejs#6577

test/parallel/test-dgram-udp6-send-default-host.js

@@ -26,7 +26,7 @@ client.on('listening', function() {
 
 client.on('message', function(buf, info) {
   const expected = toSend.shift().toString();
-  assert.ok(buf.toString() === expected, 'message was received correctly');
+  assert.strictEqual(buf.toString(), expected);
 
   if (toSend.length === 0) {
     client.close();