JSFUnit doesn't have Trinidad support yet. How fast we get there will depend on how much help we get from the community.
It is possible to roll your own Ajax support. I have a rough outline of how to do this here:
If you come up with something good for Trinidad support, please consider donating it back to JSFUnit. Otherwise, it will be awhile before you see support for it.
As for alternatives, I don't know of anything that does a good job of testing Trinidad-based applications. You could try a black-box approach using HtmlUnit. If you do, please let me know how it turns out. I'm considering a switch from HttpUnit to HtmlUnit for the JSFClientSession.