Hi Knut,
There is one, but not fully functional. What I mean is that there is a regression test suite. It can be readily run when you compile the program yourself. It is eg automatically run after every commit to avoid regressions. Unfortunately, there is no mechanism to easily run it when you install binaries. Actually, the tests are not even shipped with binaries.
On the brighter note: there should be one, and it's not going to be difficult to provide it building upon the existing mechanism. I've put your request to the bugtracker:
https://github.com/grzegorzmazur/yacas/issues/167 . Should be ready by the next release.
Thanks,
Grzesiek