I agree that it's not a great idea to try to cram or study for job interview tests. I think it will just stress you out more, which will do far more damage to your performance in the test/interview than you could hope to recoup from the studying itself.
Programming languages like C++ are massive and the scope of potential things you could be asked on a test is so huge that you're unlikely to be able to study in such a short period of time enough of the language to raise your overall bar of knowledge significantly enough to matter. You could try to focus your studying on what you think might be asked, or what you think you need to work on personally, but you could just as well never see any questions on those topics and have wasted your time. Plus, this is assuming that their "C++ test," focuses exclusively on syntactic or lexical details of the languages alone, and not the broader domain of problem solving with C++ (which is actually far more likely).
The major risk with studying for something like this is a false demonstration of knowledge. Most of these tests are designed to query what you know and what/how you can reasona about problems with that knowledge. Cramming can tend to introduce a superficial level of knowledge, so you may be able to pass questions on the test but in the follow-up interview you may not be able to speak sufficiently well about your answers to prove that you really have this knowledge internalized. This can very easily lead to the interviewers wondering how you could bomb the interview so spectacularly while still passing the test, which could lead them to conclude that you may have crammed a bunch of superficial knowledge into your brain (or you cheated, or whatever; the point is most of the conclusions they could draw would be bad for you).
Saying you don't know something is not a weakness. It can, in fact, be a difficult-to-find strength in a candidate. Obviously you will have to know some minimum amount of stuff, but it's not wrong to not know something. A common interview tactic is to ask follow-up questions until you reach the end of the candidates knowledge about a topic, to see how deep their knowledge goes, for example.
Take Apoch's advance: relax.