You said "testing", so I'm assuming you're making a test post, and part of test posting would be seeing if such a post federates.
I've got a lot of stuff I hope is federating properly, but in spite of being not too lazy to make it, I'm still too lazy to make accounts on other sites to see if it's working.
One thing with newer versions of mastodon is that it supports websockets for communications as well as the legacy APIs. That was one feature I brought over into unfathomably because it made sense on an app side but also as the link between the front end and back end.