I left it in the same directory as libtinymail-test (less clutter), added a configure flag. It's basically a new GUnit suite testing only public interfaces and not pulling libraries from current build.
I have packages from 2006-11-11 installed. I had to comment out tny-stream-test and tny-msg-test because they wouldn't compile. tny_mime_part_get_parts was missing for example. Can it be that it has been added since then? In that case it was successfully detected..